MB-8955 Prevent ExTask starvation: order by taskId

Change-Id: Ib75d92a1e64ca85843e688ebe23579841c23f4d7
Reviewed-on: http://review.couchbase.org/28550
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Chiyoung Seo <chiyoung.seo@gmail.com>