mon: OSDMonitor: smaller transactions when rebuilding full versions

Otherwise, for considerably sized rebuilds, the monitor will not only
consume vast amounts of memory, but it will also have troubles committing
the transaction.  Anyway, it's also a good idea to adjust transactions to
the granularity we want, and to be fair we care that each rebuilt full map
gets to disk, even if subsequent full maps don't (those can be rebuilt
later).

Fixes: #6323

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
mon: OSDMonitor: update latest_full while rebuilding full maps

Not doing so will make the monitor rebuild the osdmap full versions, even
though they may have been rebuilt before, every time the monitor starts.

This mostly happens when the cluster is left in an unhealthy state for
a long period of time and incremental versions build up.  Even though we
build the full maps on update_from_paxos(), not updating 'full_latest'
leads to the situation initially described.

Fixes: #6322

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