OSD: move backfill_request_timer cleanup to OSDService::cleanup

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: rename timer to tick_timer

Only used for scheduling ticks - we should keep it
that way.

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: lookup_lock_raw_pg is dead

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: must always check for is_stopping after locking osd_lock

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

Reorder teardown:
- pgs
- queues/threadpools
- persist superblock
- filestore
- timers
- messengers

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