mon: PaxosService: cache {first,last}_committed versions

Avoid unnecessary overhead from looking them up on-disk every time.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: Monitor: update PaxosService's cached values before update_from_paxos

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