ReplicatedPG: replace backfill_pos with last_backfill_started

ReplicatedPG: replace backfill_pos with last_backfill_started

last_backfill_started reflects what pinfo.last_backfill will be
once all currently outstanding backfills complete.  backfill_pos
was tricky since we couldn't correctly inialize it without
doing the first backfill scan pair.

In recover_backfill, we rescan from last_backfill_started rather
than from backfill_pos.  This ensures that we capture all clones
created between last_backfill_started and what previously had been
backfill_pos without special handling in make_writeable.  The main
downside is that we will tend to "rescan" last_backfill_started.

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