qnd: query cache triggers - added support for cases where the invalidation key of a row can change (e.g. if the puser id of kuser changes from A to B, both A and B should be invalidated)

git-svn-id: svn+ssh://kelev.kaltura.com/usr/local/kalsource/backend/server/tags/gemini_2012_10_22@92440 6b8eccd3-e8c5-4e7d-8186-e12b5326b719