mon: mdsmon 9794 fix && paxos async commit while electing fix

This branch contains two different patches fixing different issues:

1. MDSMonitors must wait for OSDMonitor to be writeable when requesting
    OSDMonitor to propose (see tracker #9794).
2. Paxos must not wake up contexts in commit_finish() IF an election has
    been triggered or quorum is gone.  This may happen if an async commit
    finishes after an election is called or quorum is lost.  Commit finishing after
    any of said events is assumed as if the commit itself happened before those
    events, which is what would have happened if the commit was still
    synchronous anyway, as the quorum loss or election effects would only be
    visible *after* we finished committing the value to disk.