mon/Paxos: don't ignore peer first_committed

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>