Remove unused cruft from multiraft.

Just expose the underlying raftpb types directly instead of creating our
own wrappers.

Also clean up some obsolete todos.
Streamline Multiraft.start select loop.

raft.MultiNode is thread-safe, so we no longer need to route everything
through our own goroutine.

Remove some unneeded generality in the transport interface.
Stopper for stopping multiple goroutines and waiting for them to complete.
Adding in DeleteArgs
Merge pull request #209 from cockroachdb/bram/add_delete

Adding in DeleteArgs
Merge pull request #210 from cockroachdb/spencerkimball/stopper

Introduce a Stopper class to encapsulate common idiom.
Merge pull request #204 from bdarnell/raft-cleanup

Raft cleanup
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.