[XF Addon] Redis Cache By Xon 2.18.0

• Restore XF2.1 support

• Fix high availability support where css updates would be sent to a possible readonly replica instead of the writable instance

• Fix XF2.3 compatibility when css caching is enabled

• XF2.3 compatibility update
  • A bug in XenForo 2.3 Beta 1 (Fixed in beta 2) prevents XenForo from using this as a caching backend without manual code patching references to Adapter\AbstractAdapter to Adapter\AdapterInterface
• Add \SV\RedisCache\Repository\Redis::get()->getRedisConnector() method, this should be used instead of \XF::app()->cache() instanceof Redis or similar.