ENG-4894: Make SiteMailbox's constructor public.
ENG-4894: Extract the mailbox from StreamSnapshotSink.

Let the owner of the sink manage the mailbox.
ENG-4894: Create a dedicated mailbox for @Rebalance data transfer.
Remove unused @UpdateTopology sysproc.
ENG-4894: Add utility method to get normal tables from catalog.

Normal tables don't include matviews and export tables.
ENG-4894: Add null mailbox check to StreamSnapshot*.
ENG-4894: Add plan fragment IDs for @BalancePartitions.
ENG-4894: Make m_streamSnapshotMb null check a precondition since it shouldn't
happen.
Fix typo.
Merge pull request #659 from VoltDB/ENG-4894

ENG-4894: StreamSnapshot* refactoring and @BalancePartitions