Notification of string changes

Author: Tim Hunt
MDL-28166 send quiz event notifications asynchronously.
This avoids the problem that you cannot send messages in transactions.

It also means that the quiz submission will not be prevented, and the
message will still be sent eventually, if any part of the messaging
system is giving intermittent errors when the student wants to submit
their quiz.


http://git.moodle.org/gw?p=moodle.git;a=commit;h=54bf1cf7dcfbaaf307ffdb2c1c4dfe296a9f6533
http://github.com/moodle/moodle/commit/54bf1cf7dcfbaaf307ffdb2c1c4dfe296a9f6533

+ 2.1 en [quiz:emailconfirmsubmission,mod_quiz]
+ 2.1 en [quiz:emailnotifysubmission,mod_quiz]