MDL-24419 (2): Moved groupings cache to get_all_groupings function

There was a static cache inside course/lib.php. I need to access this
information in other places, so to avoid making two queries, I am
moving the cache into the groups_get_all_groupings function instead.

lib/setup.php

@@ -342,6 +342,14 @@ global $OUTPUT;
  */
 global $MCACHE;
 
+/**
+ * Cache used within grouplib to cache data within current request only.
+ *
+ * @global object $GROUPLLIB_CACHE
+ * @name $GROUPLIB_CACHE
+ */
+global $GROUPLIB_CACHE;
+
 /**
  * Full script path including all params, slash arguments, scheme and host.
  *