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

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.

Note that this does not introduce any compatibility change.  This change
*only* affects the provider.  The key difference is that at the end
of the sync, the provide will set version to the latest version, and
not the version from the start of the sync (as was done previously).

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