ENG-4800: Send replicated table data once for elastic join snapshot.

Pick one site to handle replicated table data transfer for a given
destination during elastic join, otherwise the same data will be sent
multiple times causing constraint violation.
ENG-4800: Remove unused task log from join producer.

During blocking join, the task log is unused, so remove it.

In fact, this is necessary to make joining site transition to running
state immediately after data migration snapshot completes. Otherwise,
the site will transition to replay state and some SP transactions can
sneak in and dummy responses will be sent back to the client.
ENG-4800: Remove replicated table snapshot code from join producer.

It's coalesced with the partitioned table migration snapshot.
Merge pull request #551 from VoltDB/ENG-4800

ENG-4800: Coalesce elastic join snapshot