mon: OSDMonitor: fix #6322 and #6323

Both tickets refer to the OSDMonitor's update_from_paxos().

#6323 was due to the in-store latest_full not being updated during the full map rebuild, thus forcing us to keep on iterating over all the incremental versions each time a monitor was restarted.

#6322 reduced the granularity of the transaction responsible for applying the rebuilt full maps to the store. By reducing the transaction's granularity nor only do we save the monitor from certain death when rebuilding hundreds of thousands of maps (due to #6323), but we also use a more appropriate granularity for this transaction -- different full versions should have different transactions.