MB-10507 Skip a deleted item for TAP BG fetch task.

If a given item is deleted on disk while the TAP BG fetch job is
being executed. We can simply ignore it because the deletion
will be replicated separately through a checkpoint cursor.

Change-Id: I8b9bd60596fbbf658d2d297e89cc04de6683d06b
Reviewed-on: http://review.couchbase.org/34694
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: David Liao <david.liao@couchbase.com>