[SPARK-4168][WebUI] web statges number should show correctly when stages are more than 1000

The number of completed stages and failed stages showed on webUI will always be less than 1000. This is really misleading when there are already thousands of stages completed or failed. The number should be correct even when only partial stages listed on the webUI (stage info will be removed if the number is too large).

Author: Zhang, Liye <liye.zhang@intel.com>

Closes #3035 from liyezhang556520/webStageNum and squashes the following commits:

d9e29fb [Zhang, Liye] add detailed comments for variables
4ea8fd1 [Zhang, Liye] change variable name accroding to comments
f4c404d [Zhang, Liye] [SPARK-4168][WebUI] web statges number should show correctly when stages are more than 1000
[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