mon/PaxosService: fix proposal waiter handling

- Cancel the propsal waiters with EAGAIN on election, etc.
- Transfer proposal waiters to the paxos commit wait list when we propose.
  This gives Paxos responsibility for doing 0 or EAGAIN or whatever so that
  we don't have to infer that at a higher layer.
- Drop the now-unused wakeup helper.

Signed-off-by: Sage Weil <sage@inktank.com>