osd/: don't block writes on degraded objects

Signed-off-by: Samuel Just <sjust@redhat.com>
PG::purge_strays: purge peer_missing as well

Signed-off-by: Samuel Just <sjust@redhat.com>
Merge remote-tracking branches 'athanatos/wip-10059', 'athanatos/wip-10150', 'athanatos/wip-10157', 'athanatos/wip-7861', 'athanatos/wip-7862' and 'athanatos/wip-8011' into wip-sam-working