MDL-56748 cache: Removes cache definition instances in purge_store

Previously a purge_store was only purging caches but it was leaving a
reference to the store instance for the cache definition.

As a result, the cache was never cleared away correctly and, in cases
where the cache store makes a TCP connection to a backend system, the
number of open sockets was increasing for each store instance.

After a complete purge, as found in the unit test reset, the cache
definitions should be both purged and removed.

This change should also lead to a drop in memory consumption for unit
tests as there will no longer be references to unused definition stores.

cache/classes/factory.php

@@ -161,6 +161,7 @@ class cache_factory {
         $factory->reset_cache_instances();
         $factory->configs = array();
         $factory->definitions = array();
+        $factory->definitionstores = array();
         $factory->lockplugins = array(); // MUST be null in order to force its regeneration.
         // Reset the state to uninitialised.
         $factory->state = self::STATE_UNINITIALISED;