mon/OSDMonitor: account for PG merging in epoch_by_pg accounting

mon/OSDMonitor: account for PG merging in epoch_by_pg accounting

After a pool has merged PGs, the epoch_by_pg accounting will refer
to osdmap epochs of PGs that no longer exist. We'll never again get
OSD beacons for these PGs, so the min epoch in epoch_by_pg will not
advance until the mon leader has restarted. The effect of this is
that osdmaps are not trimmed after a pool has undergone PG merging,
until the mon leader restarts. To fix, we unconditionally resize
epoch_by_pg to the pg_num of the pool during each beacon report.

Fixes: https://tracker.ceph.com/issues/48212
Signed-off-by: Dan van der Ster <daniel.vanderster@cern.ch>