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?

Yes, just like it is in `get()`.

> 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().
>
> If I do array_flip at the initialization, needed operations with the array seem to be faster. 

I've kind of got this into my TODO list for after this is merged, just so we don't have to exchange yet more comments here ... Leave it as is for now.