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>
(cherry picked from commit 4ac1570c5cdcd6556dc291cc6d7878fd92d343ae)
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>
(cherry picked from commit 81983bab3630520d6c7ee9b7e4a747bc17b8c5c3)
mon: OSDMonitor: multiple rebuilt full maps per transaction

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
(cherry picked from commit 0d20cae0be701c5b6151a26ee5e4fe24d89aa20a)