SPARK-2250: show stage RDDs in UI

Author: Neville Li <neville@spotify.com>

Closes #1188 from nevillelyh/neville/ui and squashes the following commits:

d3ac425 [Neville Li] SPARK-2250: show persisted RDD in stage UI
f075db9 [Neville Li] SPARK-2035: show call stack even when description is available
Simplify locking behavior on unrollMemoryMap

This gets rid of the need to synchronize on unrollMemoryMap in
addition to putLock. Now we require all accesses on unrollMemoryMap
to synchronize on putLock, including when the executor releases the
unroll memory after a task ends.
[SPARK-2464][Streaming] Fixed Twitter stream stopping bug

Stopping the Twitter Receiver would call twitter4j's TwitterStream.shutdown, which in turn causes an Exception to be thrown to the listener. This exception caused the Receiver to be restarted. This patch check whether the receiver was stopped or not, and accordingly restarts on exception.

Author: Tathagata Das <tathagata.das1565@gmail.com>

Closes #1577 from tdas/twitter-stop and squashes the following commits:

011b525 [Tathagata Das] Fixed Twitter stream stopping bug.
Merge branch 'master' of github.com:apache/spark into them-rdd-memories