PG::calc_acting: consider newest_update_osd when choosing backfill peers

We must include newest_update_osd->second.log_tail when considering backfill
peers because in GetLog we will request logs back to the min last_update over
our acting_backfill set.  This will result in our log being extended as far
backwards as necessary to pick up any peers which can be log recovered by the
union of newest_update_osd's log and that of the chosen primary.

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