osd/OSD: fix track_pools_and_pg_num_changes on mapgaps

osd/OSD: fix track_pools_and_pg_num_changes on mapgaps

* track_pools_and_pg_num_changes call is moved before the superblock updates to
  know if we have any OSDMaps *before* trimming them (if possible).
  This is used to identify map gaps.

* track_pools_and_pg_num_changes inner loop is moved into
  _track_pools_and_pg_num_changes.

* lastmap now starts with the newest_map we have, even on mapgap.
  Previously, on mapgaps, we would (falsly) assume that this is the first start
  of this OSD and would ignore the state before the gap. This case
  is fully described in the tracker below:

  Fixes: https://tracker.ceph.com/issues/63881
Signed-off-by: Matan Breizman <mbreizma@redhat.com>