[SPARK-7469] [SQL] DAG visualization: show SQL query operators

The DAG visualization currently displays only low-level Spark primitives (e.g. `map`, `reduceByKey`, `filter` etc.). For SQL, these aren't particularly useful. Instead, we should display higher level physical operators (e.g. `Filter`, `Exchange`, `ShuffleHashJoin`). cc marmbrus

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**Before**
<img src="https://issues.apache.org/jira/secure/attachment/12731586/before.png" width="600px"/>
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**After** (Pay attention to the words)
<img src="https://issues.apache.org/jira/secure/attachment/12731587/after.png" width="600px"/>
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Author: Andrew Or <andrew@databricks.com>

Closes #5999 from andrewor14/dag-viz-sql and squashes the following commits:

0db23a4 [Andrew Or] Merge branch 'master' of github.com:apache/spark into dag-viz-sql
1e211db [Andrew Or] Update comment
0d49fd6 [Andrew Or] Merge branch 'master' of github.com:apache/spark into dag-viz-sql
ffd237a [Andrew Or] Fix style
202dac1 [Andrew Or] Make ignoreParent false by default
e61b1ab [Andrew Or] Visualize SQL operators, not low-level Spark primitives
569034a [Andrew Or] Add a flag to ignore parent settings and scopes