[SPARK-3795] Heuristics for dynamically scaling executors

This is part of a bigger effort to provide elastic scaling of executors within a Spark application ([SPARK-3174](https://issues.apache.org/jira/browse/SPARK-3174)). This PR does not provide any functionality by itself; it is a skeleton that is missing a mechanism to be added later in [SPARK-3822](https://issues.apache.org/jira/browse/SPARK-3822).

Comments and feedback are most welcome. For those of you reviewing this in detail, I highly recommend doing it through your favorite IDE instead of through the diff here.

Author: Andrew Or <andrewor14@gmail.com>
Author: Andrew Or <andrew@databricks.com>

Closes #2746 from andrewor14/scaling-heuristics and squashes the following commits:

8a4fdaa [Andrew Or] Merge branch 'master' of github.com:apache/spark into scaling-heuristics
e045df8 [Andrew Or] Add warning message (minor)
dfa31ec [Andrew Or] Fix tests
c0becc4 [Andrew Or] Merging with SPARK-3822
4784f93 [Andrew Or] Reword an awkward log message
181f27f [Andrew Or] Merge branch 'master' of github.com:apache/spark into scaling-heuristics
c79e907 [Andrew Or] Merge branch 'master' of github.com:apache/spark into scaling-heuristics
4672b90 [Andrew Or] It's nano time.
a6a30f2 [Andrew Or] Do not allow min/max executors of 0
c60ec33 [Andrew Or] Rewrite test logic with clocks
b00b680 [Andrew Or] Fix style
c3caa65 [Andrew Or] Merge branch 'master' of github.com:apache/spark into scaling-heuristics
7f9da14 [Andrew Or] Factor out logic to verify bounds on # executors (minor)
f279019 [Andrew Or] Add time mocking tests for polling loop
685e347 [Andrew Or] Factor out clock in polling loop to facilitate testing
3cea7f7 [Andrew Or] Use PrivateMethodTester to keep original class private
3156d81 [Andrew Or] Update comments and exception messages
92f36f9 [Andrew Or] Address minor review comments
abdea61 [Andrew Or] Merge branch 'master' of github.com:apache/spark into scaling-heuristics
2aefd09 [Andrew Or] Correct listener behavior
9fe6e44 [Andrew Or] Rename variables and configs + update comments and log messages
149cc32 [Andrew Or] Fix style
254c958 [Andrew Or] Merge branch 'master' of github.com:apache/spark into scaling-heuristics
5ff829b [Andrew Or] Add tests for ExecutorAllocationManager
19c6c4b [Andrew Or] Merge branch 'master' of github.com:apache/spark into scaling-heuristics
5896515 [Andrew Or] Move ExecutorAllocationManager out of scheduler package
9ca8945 [Andrew Or] Rewrite callbacks through the listener interface
5e336b9 [Andrew Or] Remove code from backend to avoid conflict with SPARK-3822
092d1fd [Andrew Or] Remove timeout logic for pending requests
1309fab [Andrew Or] Request executors by specifying the number pending
8bc0e9d [Andrew Or] Add logic to expire pending requests after timeouts
b750ee1 [Andrew Or] Express timers in terms of expiration times + remove retry logic
7f8dd47 [Andrew Or] Merge branch 'master' of github.com:apache/spark into scaling-heuristics
9d516cc [Andrew Or] Bug fix: Actually trigger the add timer / add retry timer
44f1832 [Andrew Or] Rename configs to include time units
eaae7ef [Andrew Or] Address various review comments
6f8be6c [Andrew Or] Beef up comments on what each of the timers mean
baaa403 [Andrew Or] Simplify variable names (minor)
42beec8 [Andrew Or] Reset whether the add threshold is crossed on cancellation
9bcc0bc [Andrew Or] ExecutorScalingManager -> ExecutorAllocationManager
2784398 [Andrew Or] Merge branch 'master' of github.com:apache/spark into scaling-heuristics
5a97d9e [Andrew Or] Log retry attempts in INFO + clean up logging
2f55c9f [Andrew Or] Do not keep requesting executors even after max attempts
0acd1cb [Andrew Or] Rewrite timer logic with polling
b3c7d44 [Andrew Or] Start the retry timer for adding executors at the right time
9b5f2ea [Andrew Or] Wording changes in comments and log messages
c2203a5 [Andrew Or] Simplify code to access the scheduler backend
e519d08 [Andrew Or] Simplify initialization code
2cc87a7 [Andrew Or] Add retry logic for removing executors
d0b34a6 [Andrew Or] Add retry logic for adding executors
9cc4649 [Andrew Or] Simplifying synchronization logic
67c03c7 [Andrew Or] Correct semantics of adding executors + update comments
6c48ab0 [Andrew Or] Update synchronization comment
8901900 [Andrew Or] Simplify remove policy + change the semantics of add policy
1cc8444 [Andrew Or] Minor wording change
ae5b64a [Andrew Or] Add synchronization
20ec6b9 [Andrew Or] First cut implementation of removing executors dynamically
4077ae2 [Andrew Or] Minor code re-organization
6f1fa66 [Andrew Or] First cut implementation of adding executors dynamically
b2e6dcc [Andrew Or] Add skeleton interface for requesting / killing executors