Add priority option to AsyncReserver class

Add priority to request_reservation() default to 0
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

Prioritize reservation handling for backfill

Signed-off-by: David Zafman <david.zafman@inktank.com>
Code Review changes for AsyncReserver
Code review fix: Use enum for priority values
MBackfillReserve: Rename prio to priority
RequestBackfillPrio: Change bool prio to unsigned priority
AsyncReserver: Remove assert in set_max() for max > 0
Code Review changes