PG,OSD: _remove_pg must remove pg keys

Instead of doing this in OSD::_remove_pg, pass a transaction
to on_removal and do it in PG.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG,OSD: mark info as backfilling in _remove_pg()

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD::clear_temp should clear snap mapper entries as well

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: removal collections will be removed inline and not queued

Signed-off-by: Samuel Just <sam.just@inktank.com>
WorkQueue: Allow WorkQueueVal to be specified with 1 type

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: don't rename pg collections, handle PGs in RemoveWQ

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