osd: update PGRecovery queue item cost to reflect object size

osd: update PGRecovery queue item cost to reflect object size

Previously, we used a static value of osd_recovery_cost (20M
by default) for PGRecovery. For pools with relatively small
objects, this causes mclock to backfill very very slowly as
20M massively overestimates the amount of IO each recovery
queue operation requires. Instead, add a cost_per_object
parameter to OSDService::awaiting_throttle and set it to the
average object size in the PG being queued.

Fixes: https://tracker.ceph.com/issues/58606
Signed-off-by: Samuel Just <sjust@redhat.com>
Signed-off-by: Sridhar Seshasayee <sseshasa@redhat.com>