I'm currently keeping these as longs. That's easy an unambiguous, but it also
feels rather primitive.
This was more work than I expected, but it doesn't seem particularly risky.
It's also potentially more efficient, since there's fewer places where we're
converting from string to long and vice versa.
Closes: https://github.com/square/okhttp/issues/2035
This is awkward because it adds another field to the cache. I've
decided to try to keep old values in the cache working. We will
probably do a change that'll disrupt the cache later, but that
is invasive and we don't need that yet.
