DNM: osd: bump the priority of PG from OSD with most work

Based on the number of pending backfill reservations and current
backfills in the primary OSD, increase the priority of the backfill
reservation requests. This will increase the chances of reservations
from the busiest OSDs to succeed and probably shorten the tail of the
recovery process.

http://tracker.ceph.com/issues/9566 Fixes: #9566

Signed-off-by: Loic Dachary <loic-201408@dachary.org>
I think we want to make backfill_load strictly less significant than the priotization based on how many replicas/shared degraded we are.  The latter should *always* be more signficiant in terms of data loss potential. 

We needn't confine ourselves to the 100/200 ranges from before necessarily; those were chosen because we had 0..255 and i was keeping it simple.