Cache_redis: Adding serialization support.

This is an alternative implementation of value serialization for Redis cache driver. It is the third idea that appeared at the discussion about https://github.com/EllisLab/CodeIgniter/pull/3181

Now the driver has an internal cache for storing keys of serialized values, in order these serialized values to be distinguished.

I tested it with the same test of mine, it looks OK.
I don't understand your comment at all ... I was just suggesting this:

    if (($index_key = array_search($id, $this->_serialized)) !== FALSE)
    {
        unset($this->_serialized[$index_key]);
        $this->_redis->sRemove(self::KEY_SET_FOR_SERIALIZATION, $id);
    }