mon: suspend proposing paxos values when flushing pending writes

We need to make sure that new paxos proposals are not started when we are waiting on paxos to finish writing a value, as we usually do this when we want to do something that would affect the paxos state or behaviour (say, start a new election, bootstrap/reset).

If we do not have a way to suspend paxos from proposing new values, we could incur in a state such that we would propose a new value just after finishing writing the 'currently-being-proposed' value to the store.  We could then end up doing paxos operations (begin, propose, accept,...) even though we have transitioned to a 'probing' state for instance -- and we can't be proposing stuff if we don't have a quorum, and all those asserts we have in the code will make sure of that.