MDL-41019 improve language caching

Includes:
* no more hacky reloads, everything is written only once and kept until cache reset
* lang menu list is now cached in MUC
* both string and lang menu caches are compatible with local caches on cluster nodes
* config-dist.php cleanup

lib/classes/string_manager_install.php

@@ -107,7 +107,6 @@ class core_string_manager_install implements core_string_manager {
                 if (isset($string['parentlanguage'])) {
                     $parent = $string['parentlanguage'];
                 }
-                unset($string);
             }
         }