[SPARK-17532] Add lock debugging info to thread dumps.

## What changes were proposed in this pull request?

This adds information to the web UI thread dump page about the JVM locks
held by threads and the locks that threads are blocked waiting to
acquire. This should help find cases where lock contention is causing
Spark applications to run slowly.
## How was this patch tested?

Tested by applying this patch and viewing the change in the web UI.

![thread-lock-info](https://cloud.githubusercontent.com/assets/87915/18493057/6e5da870-79c3-11e6-8c20-f54c18a37544.png

)

Additions:
- A "Thread Locking" column with the locks held by the thread or that are blocking the thread
- Links from the a blocked thread to the thread holding the lock
- Stack frames show where threads are inside `synchronized` blocks, "holding Monitor(...)"

Author: Ryan Blue <blue@apache.org>

Closes #15088 from rdblue/SPARK-17532-add-thread-lock-info.

(cherry picked from commit 2dc04808)
Signed-off-by: Reynold Xin <rxin@databricks.com>