Optimize the vtworker clones to pack N inserts together.

That should make inserts N times for efficient. N is set to 10 by default.
Closing the source RPC connection in clone workers.
Merge branch 'master' into replication
Reworking the comment to hopefully be clearer.
Merge pull request #205 from youtube/replication

Optimize the vtworker clones to pack N inserts together.