Consolidate sending and ack tracking threads used by StreamSnapshotDataTargets
to one per node.

The will only be one sending thread and one ack tracking thread per node for
stream snapshot. They share the same mailbox. All the StreamSnapshotDataTargets
will pass work to these two threads.
Consolidate StreamSnapshotSinks used during join to one per site.

Each sink will be receiving data from multiple sources.
Merge branch 'master' into stream-refactor

Resolved conflicts:
	src/frontend/org/voltdb/iv2/ElasticJoinProducer.java
	src/frontend/org/voltdb/rejoin/StreamSnapshotSink.java
Merge pull request #602 from VoltDB/stream-refactor

Consolidate threads used for stream snapshot during join