Finish implementing cache handlers

[seliopou]

Time parsing and comparison have not yet been implemented, mostly due to hesitance in adding calendarlib as a dependency. It may be worthwhile considering ptime for this use case as well.

The decision nodes in question are v3h11, v3h12, and v3l13.

[tmcgilchrist]

I've done h11 and h12 here but using calendarlib rather than ptime. What were your concerns about calendarlib? I settled for it cause it had the closest function to what I'd use in Haskell of_string : string -> string -> Time.t thought ideally it'd be an option of time rather than exceptions.

[seliopou]

This was done a while ago, closing.