[ml] [docs] Add a overview documentation for Distance Metrics

[FLINK-1933] [ml] Moves metrics.distances package to org.apache.flink.ml

This closes #629.

This closes #636.

For a more consistent behaviour.

This closes #655

The watermarks of the NettyServer, which is responsible for data transfers, were
not configured correctly. Instead they were configured for the NettyClient, which
does need this fine-grained control as it does not transfer a lot of data.

Previously, the default watermarks matched our desired watermarks of 64k (for our
default page size) by chance.

This improves tracking of jobs.

Closes #654

This closes #627

This closes #641

moved the migration guide section to the end of Gelly guide;
made a few corrections in the migration guide text

This closes #600

This closes #650

This closes #626

[FLINK-1855] [streaming] Add WordCount class into WindowWordCount and SocketTextStreamWordCount example jars

Closes #644

Moved corresponding test to flink-contrib
Removed unnecessary print

Closes #581

Streams back the results from the job to the client via a TCP socket

This is a squash of the following commits:

[FLINK-1883][gelly] Added Min Vertex Id Propagation library and example

[FLINK-1883][gelly] Renamed algorithm to Connected Components

[FLINK-1883][gelly] Made the CC library method match Spargel

[FLINK-1883][gelly] Polished the CC with Randomised Edges test

Closes #596

The previously introduced SoftReferences to store archived
ExecutionGraphs cleared old graphs in a non-transparent order.

This closes #639.

This closes #621
This closes #442

Closes #630

Closes #634

 - Checkpoints can be configured to have different sets of tasks
   that triggering the checkpoint barriers, that acknowledging the checkpoint,
   and that require checkpoint confirmations.

 - A configurable number of successful chckpoints can be retained

 - Checkpoints time out after a certain time, if not acknowledged (prevent resource leaks)

 - Checkpoints are robust to lost messages and out of order acknowledging.

[streaming] Change 'getParallelism()' signature to return a primitive to avoid failed equality checks.

  1. TDD reception with eager acknowledgement and
  2. TDD instantiation with a subsequent state update message.

This closes #622

[FLINK-1923] [runtime] Replaces asynchronous logging with synchronous logging using grizzled-slf4j wrapper for Scala.

This closes #628

This closes #631

[yarn] [python] Adds -Xms start option for yarn, fixes python test case to overwrite files, fixes python docs

Fixes failing yarn test cases

This closes #637

This closes #35