osd/: Integrate SnapMapper with OSD

- SnapTrimmer now uses SnapMapper to get the next object to trim
- Entries for a snap are implicitely removed from SnapMapper when
the last object is trimmed, so no need for the adjust_local_snaps
logic.
- Scrub now compares the object_info snaps set on the object attr
with the version stored in the SnapMapper.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG: update purged_snaps in merge_log

Signed-off-by: Samuel Just <sam.just@inktank.com>