ENG-4257: Check partition count in command log on replay after elastic join.

The truncation snapshot created at the time of join will include both
the old partition count and the new partition count. Restore agent
should parse this out and pass the latter partition count to the
command reinitiator.
ENG-4257: If serialized params is null, use the params to get the
partition param on replay.
Merge pull request #544 from VoltDB/ENG-4257

ENG-4257: Handle partition count change in truncation snapshot on replay