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.
`$result` will be an integer, you need to return boolean ... Don't cache it, you know when to return TRUE and when FALSE, and therefore you don't need a variable at all.

Also, you've ignored the second part of the expression that I gave you. Admittedly, I was wrong with my initial suggestion that you wouldn't need anything in `delete()` and I had also suggested `array_search()` here while `in_array()` would be better ... Still, if you are going to execute `sRemove()`, it's better to check the `_serialized` list first instead of a blind call.