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 should just be one extra allocation. This may be a concern, but I think we should wait and see how much we're allocating in the steady state. If we're only making a few allocations per proposal we can change this, but if there are a lot then it may not be worth changing this.