MemCache: Initial API structure.

@davidfowl @GrabYourPitchforks @lodejard @DamianEdwards @yishaigalatzer 
The current (ASP.NET) behavior allows you to specify both an absolute and a sliding expiration.  Whenever an item is accessed, the new absolute expiration becomes MAX(old absolute expiration, current time + sliding expiration).  People seem to like this behavior, so we may want to consider how it can work going forward.