osd/PeeringState.cc: skip peer_purged when discovering all missing

osd/PeeringState.cc: skip peer_purged when discovering all missing

We hit a couple of bugs because in discover_all_missing() we send
pg_query to an OSD that was marked stray and already got purged. This results
in a state machine crash on the purged OSD. Fix this by skipping any
purged peers.

Fixes: https://tracker.ceph.com/issues/41317
Fixes: https://tracker.ceph.com/issues/40963
Signed-off-by: Neha Ojha <nojha@redhat.com>
(cherry picked from commit 4960f579a234f9984d73767fde073c419e884c17)

Conflicts:
	src/osd/PeeringState.cc
- file does not exist in mimic; made the changes manually in
  src/osd/PG.cc