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.
    const SERIALIZED_SET_KEY = '_ci_redis_serialized';

My (updated) comment [here](https://github.com/EllisLab/CodeIgniter/pull/3181/files#r16169812) was about giving the *constant* a better name.