MDL-22052 Improved get_string caching - dirty on-disk cache detection
MDL-22052 Improved get_string caching - dirty on-disk cache detection

If the string is not found, core_string_manager tries to re-fetch master
sources. This prevents false debugging messages in case when the on-disk
cache is dirty. Thanks to Eloy, Tim and Petr for the ideas.

dilawar/moodle