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.
... I was suggesting to *move* the `sRemove()` call inside the `if` statement, where `unset()` is called. Haven't said anything about `array_search()`, I don't know why you're talking about that.