mon/PGMap: invalidate cached min_last_epoch_clean from new-style pg keys

We were only invalidating the cache from the legacy apply_incremental(),
which is no longer called on modern clusters.

Fixes: #9987
Signed-off-by: Sage Weil <sage@redhat.com>
mon/PGMap and PGMonitor: update last_epoch_clean cache from new osd keys

We were only invalidating the cached value from apply_incremental, which
is no longer called on modern clusters.

Fix this by storing the update epoch in the key as well (it is not part
of osd_stat_t).

Backport: giant, firefly, dumpling(?)
Fixes: #9987
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2850 from ceph/wip-9987

mon: fix min_last_epoch_started tracking

Reviewed-by: Joao Eduardo Luis <joao@redhat.com>