MemCache: Initial API structure.

@davidfowl @GrabYourPitchforks @lodejard @DamianEdwards @yishaigalatzer 
Not sure how useful a Keys property is since items can disappear during enumeration, so you can't actually rely on the item being present.  We should consider making the collection fully enumerable (IEnumerable[KeyValuePair[...]]) or not enumerable at all.