mon/Paxos: don't ignore peer first_committed

We go to the effort of keeping a map of the peer's first/last committed
so that we can send the right commits during the first phase of paxos,
but we forgot to record the first value.  This appears to simply be an
oversight.  It is mostly harmless; it just means we send extra states
that the peer already has.

Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>
mon/Paxos: update first_committed when we trim

The Paxos::trim() -> ::trim_to() path trims old states but does not
update first_committed.  This misinforms later paxos rounds such that
peers think they can participate and end up with COMMIT messages
following the COLLECT/LAST exchange that are for future commits they
can't do anything with and then crash out when they get the BEGIN:

mon/Paxos.cc: 557: FAILED assert(begin->last_committed == last_committed)

Fixes: #4879
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>