MB-9693 Return ENGINE_SUCCESS if an item is updated during bg fetch

If an item (that might not exist before) is inserted while we are
processing a bg fetch request for its metadata, we should not
return any error codes, but instead ENGNINE_SUCCESS, so that the
memcached worker thread can return that item's metadata to the
client.

Change-Id: I2f9749d422aef305764783d363d11f9921f186dd
Reviewed-on: http://review.couchbase.org/32472
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>