OSD: don't rename pg collections, handle PGs in RemoveWQ

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: add pg deletion cancelation

DeletingState now allows _create_lock_pg() to attempt to cancel
pg deletion.

PG::init() must mark the PG as backfill iff we stopped a deletion.

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd_internals/pg_removal.rst: update for pg resurrection

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