osd, pg: implement force_recovery

osd, pg: implement force_recovery

This commit implements the MOSDForceRecovery handler along with
all required code to have PGs processed in desired order
(PGs with force_recovery/force_backfill first).
Obviously it's not going to work cluster-wide and OSDs that are
not affected - but may affect affected OSDs - may cut into PG
recovery queue and cause PGs with force_* flags to get recovered
or backfilled later than expected, but still way earlier than
without it.

Signed-off-by: Piotr Dałek <piotr.dalek@corp.ovh.com>