Author: Ryan Wyllie
MDL-56139 message: ajax poll for new messages in message area
http://git.moodle.org/gw?p=moodle.git;a=commit;h=fb1469d84f409becbd8b24cedaa5b936622052f6
http://github.com/moodle/moodle/commit/fb1469d84f409becbd8b24cedaa5b936622052f6
+ 3.2 en [cachedef_message_last_created,core_cache]