MDL-38565 cache: session caches now function as expected

This change is a large change to the way sessions are handled
within MUC after it was discovered that session did not function
as expected when any store other than the default session store
was being used.
As part of this change the session loader has been largely
customised in order to consolidate session data for the loader.
The unit tests have also being greatly increased to provide
better coverage for sessions.

cache/classes/helper.php

@@ -244,10 +244,9 @@ class cache_helper {
             if ($definition->invalidates_on_event($event)) {
                 // OK at this point we know that the definition has information to invalidate on the event.
                 // There are two routes, either its an application cache in which case we can invalidate it now.
-                // or it is a session cache in which case we need to set something to the "Event invalidation" definition.
-                // No need to deal with request caches, we don't want to change data half way through a request.
-                if ($definition->get_mode() === cache_store::MODE_APPLICATION) {
-                    $cache = $factory->create_cache($definition);
+                // or it is a persistent cache that also needs to be invalidated now.
+                if ($definition->get_mode() === cache_store::MODE_APPLICATION || $definition->should_be_persistent()) {
+                    $cache = $factory->create_cache_from_definition($definition->get_component(), $definition->get_area());
                     $cache->delete_many($keys);
                 }