PGBackend: add interfaces for stashing removed objects

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd_types: pg_log_t record oldest version to which we can rollback

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd_types: hook ObjectModDesc into pg_log_entry_t

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