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.

It's returned from SubmitCommand and ChangeGroupMembership. We have tests that make use of these channels, but only tests so far. I don't know if we'll have a non-test use for channel returned by SubmitCommand (since we can use the response cache instead), but the one for ChangeGroupMembership will probably be useful since it fires at the correct time and doesn't have a response cache alternative.