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.
On another thought ... you don't need any additions to `delete()` at all.

If the call fails, you don't need to update anything (you're ignoring that currently).
If it doesn't ... who cares, `get()` will return FALSE for a non-existing item.