Remove WP Dependencies

When using the Redis object cache with batcache, it was failing because it was trying to make use of WP core methods that were not available when batcache fires. To this end the pull request does a few things:

- Removes Predis as an option as discussed
- Switches to using the internal PHP serializer in the PECL extension
- All remaining WP function dependencies are removed or handled gracefully with fallbacks.