ENG-5916: Command log replay end of log handling changes.

Simplified ReplaySequencer by removing EOL message for each partition, only the
MPI sends EOL message to SPIs now, which indicates EOL of all partitions.

Add poison flag to MpReplayAckMessage to indicate MPs with missing
sentinels.
Merge pull request #1157 from VoltDB/ENG-5916

ENG-5916: Command log replay end of log handling changes.