MB-9107: During Backfill don't read resident values off disk

We remove in-memroy backfilling in a previous commit and this
can cause backfilling to be slow if most of the items are in
memory. This change allows us to skip reading values off of disk
if the values already exist in memory.

Change-Id: I51aad158e9bab15cc8fb78f3c140c1d87c224101
Reviewed-on: http://review.couchbase.org/29115
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>