osd: prioritize recovery for degraded pgs

Do degraded PGs first by queueing in front

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