ReplicatedPG/Backend: shuffle over submit_push_*/build_push_op

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG/PGBackend: add method to get an ObjectContext out of attrs

This will be used by PGBackend implementers to get the SnapSet
and ObjectInfo for newly recovered objects on the primary.

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