Return a channel from MultiRaft.{SubmitCommand,ChangeGroupMembership}

This channel can be used to wait for the proposed action to be
committed.

Adding this wait to TestMembershipChange demonstrated that the test
wasn't actually doing much. Some of the issues have been fixed
but the test is disabled until we make some decisions about the
group membership protocol.

I take it this is a string so it can serve as a map key?