MDL-44436 context: Context IDs are forced to be int

The method context::insert_context_record() sets the context::$id to
an int, while when we create an instance, the $id is often read from
a DB $record object, and is a string. The ID should always be an int.

lib/accesslib.php

@@ -5085,7 +5085,7 @@ abstract class context extends stdClass implements IteratorAggregate {
      * @param stdClass $record
      */
     protected function __construct(stdClass $record) {
-        $this->_id           = $record->id;
+        $this->_id           = (int)$record->id;
         $this->_contextlevel = (int)$record->contextlevel;
         $this->_instanceid   = $record->instanceid;
         $this->_path         = $record->path;