ReplicatedPG: recover_backfill: don't prematurely adjust last_backfill

ReplicatedPG: recover_backfill: don't prematurely adjust last_backfill

We can't adjust last_backfill to object x until x has been fully
backfilled.  pending_backfill_updates contains all those backfills
started, but which have not yet been reflected in pinfo.last_update.
backfills_in_flight contains those backfills which have not yet
completed.  Thus, we can adjust last_update to the largest entry
in pending_backfill_updates not in backfills_in_flight.

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