osd/: update peer_missing and local missing if we write a degraded object

Signed-off-by: Samuel Just <sjust@redhat.com>
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>