osd: move osd_stat into OSDService

Signed-off-by: Sage Weil <sage@inktank.com>
osd: prioritize backfill based on *how* degraded

Instead of prioritizing all degraded PGs equally, prioritize those that
have the fewest acting members (the most degraded).

Drop the priority constants.

Define our max priority independently of AsyncReserver, which uses a host
unsigned.

Signed-off-by: Sage Weil <sage@inktank.com>