[SPARK-611] Display executor thread dumps in web UI

This patch allows executor thread dumps to be collected on-demand and viewed in the Spark web UI.

The thread dumps are collected using Thread.getAllStackTraces().  To allow remote thread dumps to be triggered from the web UI, I added a new `ExecutorActor` that runs inside of the Executor actor system and responds to RPCs from the driver.  The driver's mechanism for obtaining a reference to this actor is a little bit hacky: it uses the block manager master actor to determine the host/port of the executor actor systems in order to construct ActorRefs to ExecutorActor.  Unfortunately, I couldn't find a much cleaner way to do this without a big refactoring of the executor -> driver communication.

Screenshots:

![image](https://cloud.githubusercontent.com/assets/50748/4781793/7e7a0776-5cbf-11e4-874d-a91cd04620bd.png)

![image](https://cloud.githubusercontent.com/assets/50748/4781794/8bce76aa-5cbf-11e4-8d13-8477748c9f7e.png)

![image](https://cloud.githubusercontent.com/assets/50748/4781797/bd11a8b8-5cbf-11e4-9ad7-a7459467ec8e.png)

Author: Josh Rosen <joshrosen@databricks.com>

Closes #2944 from JoshRosen/jstack-in-web-ui and squashes the following commits:

3c21a5d [Josh Rosen] Address review comments:
880f7f7 [Josh Rosen] Merge remote-tracking branch 'origin/master' into jstack-in-web-ui
f719266 [Josh Rosen] Merge remote-tracking branch 'origin/master' into jstack-in-web-ui
19707b0 [Josh Rosen] Add one comment.
127a130 [Josh Rosen] Update to use SparkContext.DRIVER_IDENTIFIER
b8e69aa [Josh Rosen] Merge remote-tracking branch 'origin/master' into jstack-in-web-ui
3dfc2d4 [Josh Rosen] Add missing file.
bc1e675 [Josh Rosen] Undo some leftover changes from the earlier approach.
f4ac1c1 [Josh Rosen] Switch to on-demand collection of thread dumps
dfec08b [Josh Rosen] Add option to disable thread dumps in UI.
4c87d7f [Josh Rosen] Use separate RPC for sending thread dumps.
2b8bdf3 [Josh Rosen] Enable thread dumps from the driver when running in non-local mode.
cc3e6b3 [Josh Rosen] Fix test code in DAGSchedulerSuite.
87b8b65 [Josh Rosen] Add new listener event for thread dumps.
8c10216 [Josh Rosen] Add missing file.
0f198ac [Josh Rosen] [SPARK-611] Display executor thread dumps in web UI