ENG-4815: Initialize command log in RealVoltDB.initialize() for elastic join.

Just like rejoin, the command log has to be initialized in
RealVoltDB.initialize() for elastic join.
ENG-4815: Add comment to command log initialization in RealVoltDB per Steve's
comment.
Merge pull request #563 from VoltDB/ENG-4815

ENG-4815: Initialize command log in RealVoltDB.initialize() for elastic join