mon: include any new paxos commits in each sync CHUNK message

We already take note of the paxos version when we begin the sync.  As
sync progresses and there are new paxos commits/txns, include those
and update last_committed, so that when sync completes we will have
a full view of everything that happened during sync.

Signed-off-by: Sage Weil <sage@inktank.com>