ENG-4835: Create a global fixed buffer pool for stream snapshot receivers.

A global buffer pool with a bounded size will be created for all
StreamSnapshotReceivers on that node for join/rejoin. The pool is passed to each
receiver via the local RejoinMessage. Once the snapshot transfer is finished,
the pool is cleared.
ENG-4835: Fix TestIv2RejoinCoordinator*.
ENG-4835: Synchronize on FixedDBBPool.clear() to avoid concurrent modification.
ENG-4835: Clear rejoin snapshot buffer pool after all sites have finished
snapshot streaming.
ENG-4835: Allow user specify the size of snapshot receive buffer pool for rejoin
using system property REJOIN_RECEIVE_BUFFER_POOL_SIZE.
Merge pull request #588 from VoltDB/ENG-4835-continue

ENG-4835: Reduce direct byte buffer usage on elastic join.
Merge branch 'elastic'