Remove CmdID from InternalRaftCommand and pass it to raft separately.

Multiraft needs a command identifier so it can track pending commands
and resubmit them when an election has caused them to be dropped.
PR feedback for InternalRaftCommand.CmdID removal.
Merge pull request #211 from bdarnell/raft-cmd-id

Remove CmdID from InternalRaftCommand and pass it to raft separately.