MDL-42016 repository: Simplify API to sync external files

Too many functions, too different parameters, unnecessary DB queries.
All repositories developed for Moodle 2.3-2.5 will continue to work.

Also get rid of DB field files_reference.lifetime, it is not used by
anybody except repository itself.

lib/phpunit/classes/util.php

@@ -201,9 +201,6 @@ class phpunit_util extends testing_util {
         events_get_handlers('reset');
         core_text::reset_caches();
         get_message_processors(false, true);
-        if (class_exists('repository')) {
-            repository::reset_caches();
-        }
         filter_manager::reset_caches();
         //TODO MDL-25290: add more resets here and probably refactor them to new core function