Fix deadlock in StreamSnapshotDataReceiver.

Only grab the buffer from the pool after receiving a message from the
mailbox. If the buffer is grabbed before receiving the message, this thread
could hold on to a buffer it may not need and other receivers will be blocked if
the pool has no more buffers left.
Check all data sinks for first block at the beginning of elastic join data transfer.
Merge pull request #590 from VoltDB/streamsnapshot-deadlock

Fix stream snapshot receiver deadlock