MDL-56139 core: changes after peer review

- No longer use the Fibonacci sequence for delaying the timeout.
  It is too aggressive.
- The backoff_timer AMD module now expects the callback AND the
  backoff function to be passed to the constructor.
- Added ability to specify polling frequency in config.php.
- Added helper function to return the cache key.
- Reworded the parameters for clarity.

message/classes/api.php

@@ -291,31 +291,29 @@ class api {
      * @param int $limitfrom
      * @param int $limitnum
      * @param string $sort
-     * @param int $createdfrom the timestamp from which the messages were created
-     * @param int $createdto the time up until which the message was created
+     * @param int $timefrom the time from the message being sent
+     * @param int $timeto the time up until the message being sent
      * @return array
      */
     public static function get_messages($userid, $otheruserid, $limitfrom = 0, $limitnum = 0,
-        $sort = 'timecreated ASC', $createdfrom = 0, $createdto = 0) {
+        $sort = 'timecreated ASC', $timefrom = 0, $timeto = 0) {
 
-        if (!empty($createdfrom)) {
+        if (!empty($timefrom)) {
             // Check the cache to see if we even need to do a DB query.
-            $cache = \cache::make('core', 'message_last_created');
-            $ids = [$otheruserid, $userid];
-            sort($ids);
-            $key = implode('_', $ids);
+            $cache = \cache::make('core', 'message_time_last_message_between_users');
+            $key = helper::get_last_message_time_created_cache_key($otheruserid, $userid);
             $lastcreated = $cache->get($key);
 
             // The last known message time is earlier than the one being requested so we can
             // just return an empty result set rather than having to query the DB.
-            if ($lastcreated && $lastcreated < $createdfrom) {
+            if ($lastcreated && $lastcreated < $timefrom) {
                 return [];
             }
         }
 
         $arrmessages = array();
         if ($messages = helper::get_messages($userid, $otheruserid, 0, $limitfrom, $limitnum,
-                                             $sort, $createdfrom, $createdto)) {
+                                             $sort, $timefrom, $timeto)) {
 
             $arrmessages = helper::create_messages($userid, $messages);
         }