mon: Paxos: update first_committed on first paxos proposal

We were adding this update to a transaction that would only be applied
on the leader.  The peons would never see a first_committed > 0.

Fixes: #5484

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>