Exposes data available in the UI as json over http.  Key points:

* new endpoints, handled independently of existing XyzPage classes.  Root entrypoint is `JsonRootResource`
* Uses jersey + jackson for routing & converting POJOs into json
* tests against known results in `HistoryServerSuite`
* also fixes some minor issues w/ the UI -- synchronizing on access to `StorageListener` & `StorageStatusListener`, and fixing some inconsistencies w/ the way we handle retained jobs & stages.

Author: Imran Rashid <irashid@cloudera.com>

Closes #4435 from squito/SPARK-3454 and squashes the following commits:

da1e35f [Imran Rashid] typos etc.
5e78b4f [Imran Rashid] fix rendering problems
5ae02ad [Imran Rashid] Merge branch 'master' into SPARK-3454
f016182 [Imran Rashid] change all constructors json-pojo class constructors to be private[spark] to protect us from mima-false-positives if we add fields
3347b72 [Imran Rashid] mark EnumUtil as @Private
ec140a2 [Imran Rashid] create @Private
cc1febf [Imran Rashid] add docs on the metrics-as-json api
cbaf287 [Imran Rashid] Merge branch 'master' into SPARK-3454
56db31e [Imran Rashid] update tests for mulit-attempt
7f3bc4e [Imran Rashid] Revert "add sbt-revolved plugin, to make it easier to start & stop http servers in sbt"
67008b4 [Imran Rashid] rats
9e51400 [Imran Rashid] style
c9bae1c [Imran Rashid] handle multiple attempts per app
b87cd63 [Imran Rashid] add sbt-revolved plugin, to make it easier to start & stop http servers in sbt
188762c [Imran Rashid] multi-attempt
2af11e5 [Imran Rashid] Merge branch 'master' into SPARK-3454
befff0c [Imran Rashid] review feedback
14ac3ed [Imran Rashid] jersey-core needs to be explicit; move version & scope to parent pom.xml
f90680e [Imran Rashid] Merge branch 'master' into SPARK-3454
dc8a7fe [Imran Rashid] style, fix errant comments
acb7ef6 [Imran Rashid] fix indentation
7bf1811 [Imran Rashid] move MetricHelper so mima doesnt think its exposed; comments
9d889d6 [Imran Rashid] undo some unnecessary changes
f48a7b0 [Imran Rashid] docs
52bbae8 [Imran Rashid] StorageListener & StorageStatusListener needs to synchronize internally to be thread-safe
31c79ce [Imran Rashid] asm no longer needed for SPARK_PREPEND_CLASSES
b2f8b91 [Imran Rashid] @DeveloperApi
2e19be2 [Imran Rashid] lazily convert ApplicationInfo to avoid memory overhead
ba3d9d2 [Imran Rashid] upper case enums
39ac29c [Imran Rashid] move EnumUtil
d2bde77 [Imran Rashid] update error handling & scoping
4a234d3 [Imran Rashid] avoid jersey-media-json-jackson b/c of potential version conflicts
a157a2f [Imran Rashid] style
7bd4d15 [Imran Rashid] delete security test, since it doesnt do anything
a325563 [Imran Rashid] style
a9c5cf1 [Imran Rashid] undo changes superceeded by master
0c6f968 [Imran Rashid] update deps
1ed0d07 [Imran Rashid] Merge branch 'master' into SPARK-3454
4c92af6 [Imran Rashid] style
f2e63ad [Imran Rashid] Merge branch 'master' into SPARK-3454
c22b11f [Imran Rashid] fix compile error
9ea682c [Imran Rashid] go back to good ol' java enums
cf86175 [Imran Rashid] style
d493b38 [Imran Rashid] Merge branch 'master' into SPARK-3454
f05ae89 [Imran Rashid] add in ExecutorSummaryInfo for MiMa :(
101a698 [Imran Rashid] style
d2ef58d [Imran Rashid] revert changes that had HistoryServer refresh the application listing more often
b136e39b [Imran Rashid] Revert "add sbt-revolved plugin, to make it easier to start & stop http servers in sbt"
e031719 [Imran Rashid] fixes from review
1f53a66 [Imran Rashid] style
b4a7863 [Imran Rashid] fix compile error
2c8b7ee [Imran Rashid] rats
1578a4a [Imran Rashid] doc
674f8dc [Imran Rashid] more explicit about total numbers of jobs & stages vs. number retained
9922be0 [Imran Rashid] Merge branch 'master' into stage_distributions
f5a5196 [Imran Rashid] undo removal of renderJson from MasterPage, since there is no substitute yet
db61211 [Imran Rashid] get JobProgressListener directly from UI
fdfc181 [Imran Rashid] stage/taskList
63eb4a6 [Imran Rashid] tests for taskSummary
ad27de8 [Imran Rashid] error handling on quantile values
b2efcaf [Imran Rashid] cleanup, combine stage-related paths into one resource
aaba896 [Imran Rashid] wire up task summary
a4b1397 [Imran Rashid] stage metric distributions
e48ba32 [Imran Rashid] rename
eaf3bbb [Imran Rashid] style
25cd894 [Imran Rashid] if only given day, assume GMT
51eaedb [Imran Rashid] more visibility fixes
9f28b7e [Imran Rashid] ack, more cleanup
99764e1 [Imran Rashid] Merge branch 'SPARK-3454_w_jersey' into SPARK-3454
a61a43c [Imran Rashid] oops, remove accidental checkin
a066055 [Imran Rashid] set visibility on a lot of classes
1f361c8 [Imran Rashid] update rat-excludes
0be5120 [Imran Rashid] Merge branch 'master' into SPARK-3454_w_jersey
2382bef [Imran Rashid] switch to using new "enum"
fef6605 [Imran Rashid] some utils for working w/ new "enum" format
dbfc7bf [Imran Rashid] style
b86bcb0 [Imran Rashid] update test to look at one stage attempt
5f9df24 [Imran Rashid] style
7fd156a [Imran Rashid] refactor jsonDiff to avoid code duplication
73f1378 [Imran Rashid] test json; also add test cases for cleaned stages & jobs
97d411f [Imran Rashid] json endpoint for one job
0c96147 [Imran Rashid] better error msgs for bad stageId vs bad attemptId
dddbd29 [Imran Rashid] stages have attempt; jobs are sorted; resource for all attempts for one stage
190c17a [Imran Rashid] StagePage should distinguish no task data, from unknown stage
84cd497 [Imran Rashid] AllJobsPage should still report correct completed & failed job count, even if some have been cleaned, to make it consistent w/ AllStagesPage
36e4062 [Imran Rashid] SparkUI needs to know about startTime, so it can list its own applicationInfo
b4c75ed [Imran Rashid] fix merge conflicts; need to widen visibility in a few cases
e91750a [Imran Rashid] Merge branch 'master' into SPARK-3454_w_jersey
56d2fc7 [Imran Rashid] jersey needs asm for SPARK_PREPEND_CLASSES to work
f7df095 [Imran Rashid] add test for accumulables, and discover that I need update after all
9c0c125 [Imran Rashid] add accumulableInfo
00e9cc5 [Imran Rashid] more style
3377e61 [Imran Rashid] scaladoc
d05f7a9 [Imran Rashid] dont use case classes for status api POJOs, since they have binary compatibility issues
654cecf [Imran Rashid] move all the status api POJOs to one file
b86e2b0 [Imran Rashid] style
18a8c45 [Imran Rashid] Merge branch 'master' into SPARK-3454_w_jersey
5598f19 [Imran Rashid] delete some unnecessary code, more to go
56edce0 [Imran Rashid] style
017c755 [Imran Rashid] add in metrics now available
1b78cb7 [Imran Rashid] fix some import ordering
0dc3ea7 [Imran Rashid] if app isnt found, reload apps from FS before giving up
c7d884f [Imran Rashid] fix merge conflicts
0c12b50 [Imran Rashid] Merge branch 'master' into SPARK-3454_w_jersey
b6a96a8 [Imran Rashid] compare json by AST, not string
cd37845 [Imran Rashid] switch to using java.util.Dates for times
a4ab5aa [Imran Rashid] add in explicit dependency on jersey 1.9 -- maven wasn't happy before this
4fdc39f [Imran Rashid] refactor case insensitive enum parsing
cba1ef6 [Imran Rashid] add security (maybe?) for metrics json
f0264a7 [Imran Rashid] switch to using jersey for metrics json
bceb3a9 [Imran Rashid] set http response code on error, some testing
e0356b6 [Imran Rashid] put new test expectation files in rat excludes (is this OK?)
b252e7a [Imran Rashid] small cleanup of accidental changes
d1a8c92 [Imran Rashid] add sbt-revolved plugin, to make it easier to start & stop http servers in sbt
4b398d0 [Imran Rashid] expose UI data as json in new endpoints