ENG-5166: Initiate rejoin on next site when the previous site finishes replay.

If the next site starts before the previous site finishes replay, it may set the
hashinator into the future and reject transactions incorrectly if the cluster is
in the middle of a join.
ENG-5166: Send hashinator config as part of the rejoin stream snapshot and
updates the hashinator on the receiving end.

The hashinator being sent comes from the MPI, so it's the most up-to-date one in
the cluster.
ENG-5166: Handle null hashinator when in legacy mode.
Merge pull request #833 from VoltDB/ENG-5166

ENG-5166: Rejoin should sync hashinator from MPI