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/interfaces.php

@@ -338,6 +338,30 @@ interface cache_is_key_aware {
     public function has_all(array $keys);
 }
 
+/**
+ * Cache store feature: keys are searchable.
+ *
+ * Cache stores can choose to implement this interface.
+ * In order for a store to be usable as a session cache it must implement this interface.
+ *
+ * @since 2.4.4
+ */
+interface cache_is_searchable {
+    /**
+     * Finds all of the keys being used by the cache store.
+     *
+     * @return array.
+     */
+    public function find_all();
+
+    /**
+     * Finds all of the keys whose keys start with the given prefix.
+     *
+     * @param string $prefix
+     */
+    public function find_by_prefix($prefix);
+}
+
 /**
  * Cache store feature: configurable.
  *