[SPARK-7120] [SPARK-7121] Closure cleaner nesting + documentation + tests

Note: ~600 lines of this is test code, and ~100 lines documentation.

**[SPARK-7121]** ClosureCleaner does not handle nested closures properly. For instance, in SparkContext, I tried to do the following:
```
def scope[T](body: => T): T = body // no-op
def myCoolMethod(path: String): RDD[String] = scope {
  parallelize(1 to 10).map { _ => path }
}
```
and I got an exception complaining that SparkContext is not serializable. The issue here is that the inner closure is getting its path from the outer closure (the scope), but the outer closure references the SparkContext object itself to get the `parallelize` method.

Note, however, that the inner closure doesn't actually need the SparkContext; it just needs a field from the outer closure. If we modify ClosureCleaner to clean the outer closure recursively using only the fields accessed by the inner closure, then we can serialize the inner closure.

**[SPARK-7120]** Also, the other thing is that this file is one of the least understood, partly because it is very low level and is written a long time ago. This patch attempts to change that by adding the missing documentation.

This is blocking my effort on a separate task #5729.

Author: Andrew Or <andrew@databricks.com>

Closes #5685 from andrewor14/closure-cleaner and squashes the following commits:

cd46230 [Andrew Or] Revert a small change that affected streaming
0bbe77f [Andrew Or] Fix style
ea874bc [Andrew Or] Fix tests
26c5072 [Andrew Or] Address comments
16fbcfd [Andrew Or] Merge branch 'master' of github.com:apache/spark into closure-cleaner
26c7aba [Andrew Or] Revert "In sc.runJob, actually clean the inner closure"
6f75784 [Andrew Or] Revert "Guard against NPE if CC is used outside of an application"
e909a42 [Andrew Or] Guard against NPE if CC is used outside of an application
3998168 [Andrew Or] In sc.runJob, actually clean the inner closure
9187066 [Andrew Or] Merge branch 'master' of github.com:apache/spark into closure-cleaner
d889950 [Andrew Or] Revert "Bypass SerializationDebugger for now (SPARK-7180)"
9419efe [Andrew Or] Bypass SerializationDebugger for now (SPARK-7180)
6d4d3f1 [Andrew Or] Fix scala style?
4aab379 [Andrew Or] Merge branch 'master' of github.com:apache/spark into closure-cleaner
e45e904 [Andrew Or] More minor updates (wording, renaming etc.)
8b71cdb [Andrew Or] Update a few comments
eb127e5 [Andrew Or] Use private method tester for a few things
a3aa465 [Andrew Or] Add more tests for individual closure cleaner operations
e672170 [Andrew Or] Guard against potential infinite cycles in method visitor
6d36f38 [Andrew Or] Fix closure cleaner visibility
2106f12 [Andrew Or] Merge branch 'master' of github.com:apache/spark into closure-cleaner
263593d [Andrew Or] Finalize tests
06fd668 [Andrew Or] Make closure cleaning idempotent
a4866e3 [Andrew Or] Add tests (still WIP)
438c68f [Andrew Or] Minor changes
2390a60 [Andrew Or] Feature flag this new behavior
86f7823 [Andrew Or] Implement transitive cleaning + add missing documentation

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