PGBackend,ReplicatedBackend: add interfaces for scanning the pg

This will be important since the erasure coded pg will have a different
on-disk format than the replicated backend.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG.cc: remove leading empty space

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG,ReplicatedPG: expose PGBackend to PG

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG,PG: adapt collection_list* users to PGBackend interface

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG,PG: use PGBackend methods for getattr

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: add debugging in recover_replicas for objects added for backfill

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