osd/: Update PGBackend users to project last_update and submit stat deltas

osd/: Update PGBackend users to project last_update and submit stat deltas

The RMW pipeline means that we don't start committing an update
immediately, so we can't update the log syncronously with
submit_transaction.  Thus, in order to pipeline writes, PG/ReplicatedPG
will need to project last_update and abstain from updating info
directly (updating info.stats was the only offender).

Signed-off-by: Samuel Just <sjust@redhat.com>