mon: use wait_for_commit to reply

mon: use wait_for_commit to reply

Once PaxosService::refresh is called, the commit is final and will not
be rolled back. We should immediately send out any replies depending on
that event. This avoids a problem where PaxosService::update_from_paxos
indicates the mons need to re-bootstrap (because of a monmap change) and
these pending reply contexts are dropped in PaxosService::restart.

The cherry on top for this change is that mon commands enjoy a shorter
RTT latency as replies are sent out faster. Using the new
bench_commit.py script:

main branch:
        min/max/mean/stddev: 0.015813/0.613919/0.031615/0.024087
        min/max/mean/stddev: 0.015737/0.255008/0.031492/0.017930
        min/max/mean/stddev: 0.014242/0.205763/0.031969/0.018022
        min/max/mean/stddev: 0.014172/0.270256/0.032070/0.021079
        min/max/mean/stddev: 0.017767/0.471187/0.032751/0.025317

this commit:
        min/max/mean/stddev: 0.010476/0.158475/0.026324/0.013662
        min/max/mean/stddev: 0.016866/0.099403/0.027938/0.010508
        min/max/mean/stddev: 0.014013/0.127512/0.026847/0.010340
        min/max/mean/stddev: 0.013098/0.172725/0.028979/0.012998
        min/max/mean/stddev: 0.016934/0.292218/0.029252/0.014904

About a 10-20% reduction in latency.

Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>