Author: David Monllao
MDL-50645 cache: New plugin_functions cache
Improves memory usage as we don't need to include all files every time we want
to see if any plugin supports a callback.
http://git.moodle.org/gw?p=moodle.git;a=commit;h=9effa1481028c85ea90e6740fb666b804a9b7161
http://github.com/moodle/moodle/commit/9effa1481028c85ea90e6740fb666b804a9b7161
+ 3.0 en [cachedef_plugin_functions,core_cache]