#3814 Use fastArray cache for extension system

This reduces repetitive array hits on the Redis or other cache as they
are stored in the php layer. The Redis hits are not slow - but the
serializing & unserializing is so this is a performance improvement

CRM/Extension/System.php

@@ -251,7 +251,10 @@ class CRM_Extension_System {
   }
 
   /**
+   * Get the cache object.
+   *
    * @return CRM_Utils_Cache_Interface
+   * @throws \CRM_Core_Exception
    */
   public function getCache() {
     if ($this->cache === NULL) {
@@ -261,6 +264,7 @@ class CRM_Extension_System {
         'name' => $cacheGroup,
         'type' => ['*memory*', 'SqlGroup', 'ArrayCache'],
         'prefetch' => TRUE,
+        'withArray' => 'fast',
       ]);
     }
     return $this->cache;