calendar: performance optimisations + behaviour fix (MDL-7416 MDL-9617)

With 2500 courses the calendar was causing around 15000 queries at login and 5000 on the home page.

- Replaced queries inside loops with single unlooped queries.
- Fixed calendar so that course events will show when the users role is assigned at a category level.
- Moved calendar_session_vars() function call out of calendar/lib.php (including a lib should not have side effects)

Author: Matt Clarkson - with some conflict resolution from MartinL

blocks/calendar_upcoming/block_calendar_upcoming.php

@@ -16,7 +16,8 @@ class block_calendar_upcoming extends block_base {
         if ($this->content !== NULL) {
             return $this->content;
         }
-
+        // Initialize the session variables
+        calendar_session_vars();
         $this->content = new stdClass;
         $this->content->text = '';