Author: Zachary Durber
MDL-23273 mod_choice: Using locks to ensure choice limits
http://git.moodle.org/gw?p=moodle.git;a=commit;h=93f6e83dce3333bd7f090800dd464ee4bca0b61f
http://github.com/moodle/moodle/commit/93f6e83dce3333bd7f090800dd464ee4bca0b61f
+ 2.7 en [cannotsubmit,mod_choice]