ENG-4699: Add shouldTruncate option to stream snapshot request config.
ENG-4699: Create default accept-all predicate for snapshot targets that don't
specify predicates.
ENG-4699: If command log is enabled, pass the command log snapshot path to the
elastic join coordinator.
ENG-4699: Update the partition count on each site as post snapshot task for
partitioned data migration stream snapshot.
ENG-4699: Use the partition count in the context instead of grabbing it from the
cartographer for native snapshot.
ENG-4699: Record the new partition count in the coalesced truncation snapshot's
digest at the end of elastic join.
Reverting "ENG-4699: Create default accept-all predicate for snapshot targets
that don't specify predicates."

Add a special case to not specify any predicate if there is only data target
with no predicate associated. This will make common case faster because it
doesn't have to evaluate the dummy expression.

This reverts commit 9ca7e17a97352ba89962cc6c214833413378eb85.
Merge pull request #527 from VoltDB/ENG-4699

ENG-4699: Coalesce truncation snapshot with data migration snapshot