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.
You mean search_array() -> in_array() replacement? Do I get it right?

If so, I could flip the array after it has been retrieved from Redis. Additionally within get() method then a replacement is to be done: in_array() -> array_key_exists().