MDL-64783 analytics: Performance improvements

- Removed redundant query to analytics_predict_samples
- Analysers API now uses recordsets to iterate through the analysable
  elements. They take the last analysed time into account.
- New method for targets so there is no need to always update the last
  analysis time. Useful for lightweight targets.

analytics/classes/manager.php

@@ -565,15 +565,19 @@ class manager {
         $models = self::get_all_models();
         foreach ($models as $model) {
             $analyser = $model->get_analyser(array('notimesplitting' => true));
-            $analysables = $analyser->get_analysables();
-            if (!$analysables) {
+            $analysables = $analyser->get_analysables_iterator();
+
+            $analysableids = [];
+            foreach ($analysables as $analysable) {
+                if (!$analysable) {
+                    continue;
+                }
+                $analysableids[] = $analysable->get_id();
+            }
+            if (empty($analysableids)) {
                 continue;
             }
 
-            $analysableids = array_map(function($analysable) {
-                return $analysable->get_id();
-            }, $analysables);
-
             list($notinsql, $params) = $DB->get_in_or_equal($analysableids, SQL_PARAMS_NAMED, 'param', false);
             $params['modelid'] = $model->get_id();