osd/PrimaryLogPG: limit the number of concurrently trimming pgs

osd/PrimaryLogPG: limit the number of concurrently trimming pgs

This patch introduces an AsyncReserver for snap trimming to limit the
number of pgs on any single OSD which can be trimming, as with backfill.
Unlike backfill, we don't take remote reservations on the assumption
that the set of pgs with trimming work to do is already well
distributed, so it doesn't seem worth the implementation overhead to get
reservations from the peers as well.

Signed-off-by: Samuel Just <sjust@redhat.com>