AsyncReserver: Remove assert in set_max() for max > 0

Signed-off-by: David Zafman <david.zafman@inktank.com>
Add priority option to AsyncReserver class

Add priority to request_reservation()
Change to map of lists by prioriry
Add priority to queue_pointers mappped type

Signed-off-by: David Zafman <david.zafman@inktank.com>
osd: prioritize recovery for degraded pgs

Three Reservation priorities from RECOVERY, BACKFILL_HIGH, BACKFILL_LOW

fixes: #4273

Signed-off-by: David Zafman <david.zafman@inktank.com>