MemCache: Initial API structure.

@davidfowl @GrabYourPitchforks @lodejard @DamianEdwards @yishaigalatzer 
I can see why that combination of UTC + sliding would give you a nice "minimum plus"... Seems like a very special case - in all the combinations it would be that either token would cause expiration, but in this combination it's that both must trigger to cause expiration.

That suggests to me even further that you'd want to provide the (absolute, timespan, sliding) all in one function call - either overloaded or with suggestive names for the rule they represent. Method doc-comments could spell out what the expiration behavior they are calling for. Calling two different functions to set the time-policy would be an error.