[SPARK-2321] Stable pull-based progress / status API

This pull request is a first step towards the implementation of a stable, pull-based progress / status API for Spark (see [SPARK-2321](https://issues.apache.org/jira/browse/SPARK-2321)).  For now, I'd like to discuss the basic implementation, API names, and overall interface design.  Once we arrive at a good design, I'll go back and add additional methods to expose more information via these API.

#### Design goals:

- Pull-based API
- Usable from Java / Scala / Python (eventually, likely with a wrapper)
- Can be extended to expose more information without introducing binary incompatibilities.
- Returns immutable objects.
- Don't leak any implementation details, preserving our freedom to change the implementation.

#### Implementation:

- Add public methods (`getJobInfo`, `getStageInfo`) to SparkContext to allow status / progress information to be retrieved.
- Add public interfaces (`SparkJobInfo`, `SparkStageInfo`) for our API return values.  These interfaces consist entirely of Java-style getter methods.  The interfaces are currently implemented in Java.  I decided to explicitly separate the interface from its implementation (`SparkJobInfoImpl`, `SparkStageInfoImpl`) in order to prevent users from constructing these responses themselves.
-Allow an existing JobProgressListener to be used when constructing a live SparkUI.  This allows us to re-use this listeners in the implementation of this status API.  There are a few reasons why this listener re-use makes sense:
   - The status API and web UI are guaranteed to show consistent information.
   - These listeners are already well-tested.
   - The same garbage-collection / information retention configurations can apply to both this API and the web UI.
- Extend JobProgressListener to maintain `jobId -> Job` and `stageId -> Stage` mappings.

The progress API methods are implemented in a separate trait that's mixed into SparkContext.  This helps to avoid SparkContext.scala from becoming larger and more difficult to read.

Author: Josh Rosen <joshrosen@databricks.com>
Author: Josh Rosen <joshrosen@apache.org>

Closes #2696 from JoshRosen/progress-reporting-api and squashes the following commits:

e6aa78d [Josh Rosen] Add tests.
b585c16 [Josh Rosen] Accept SparkListenerBus instead of more specific subclasses.
c96402d [Josh Rosen] Address review comments.
2707f98 [Josh Rosen] Expose current stage attempt id
c28ba76 [Josh Rosen] Update demo code:
646ff1d [Josh Rosen] Document spark.ui.retainedJobs.
7f47d6d [Josh Rosen] Clean up SparkUI constructors, per Andrew's feedback.
b77b3d8 [Josh Rosen] Merge remote-tracking branch 'origin/master' into progress-reporting-api
787444c [Josh Rosen] Move status API methods into trait that can be mixed into SparkContext.
f9a9a00 [Josh Rosen] More review comments:
3dc79af [Josh Rosen] Remove creation of unused listeners in SparkContext.
249ca16 [Josh Rosen] Address several review comments:
da5648e [Josh Rosen] Add example of basic progress reporting in Java.
7319ffd [Josh Rosen] Add getJobIdsForGroup() and num*Tasks() methods.
cc568e5 [Josh Rosen] Add note explaining that interfaces should not be implemented outside of Spark.
6e840d4 [Josh Rosen] Remove getter-style names and "consistent snapshot" semantics:
08cbec9 [Josh Rosen] Begin to sketch the interfaces for a stable, public status API.
ac2d13a [Josh Rosen] Add jobId->stage, stageId->stage mappings in JobProgressListener
24de263 [Josh Rosen] Create UI listeners in SparkContext instead of in Tabs:
[SPARK-4088] [PySpark] Python worker should exit after socket is closed by JVM

In case of take() or exception in Python, python worker may exit before JVM read() all the response, then the write thread may raise "Connection reset" exception.

Python should always wait JVM to close the socket first.

cc JoshRosen This is a warm fix, or the tests will be flaky, sorry for that.

Author: Davies Liu <davies@databricks.com>

Closes #2941 from davies/fix_exit and squashes the following commits:

9d4d21e [Davies Liu] fix race