MDL-48468 cache: Import Redis cachestore

This code was originally taken from; https://github.com/durana/moodle-cachestore_redis
Special thanks to Mark Nielsen and Adam Durana for their work on this.

Since the original import;
- Mode session support has been added.
- Some cleanup of names has been made.
- Data guarantee and references support was added.

lib/classes/plugin_manager.php

@@ -1744,7 +1744,7 @@ class core_plugin_manager {
             ),
 
             'cachestore' => array(
-                'file', 'memcache', 'memcached', 'mongodb', 'session', 'static', 'apcu'
+                'file', 'memcache', 'memcached', 'mongodb', 'session', 'static', 'apcu', 'redis'
             ),
 
             'calendartype' => array(