Expiry Refactor part 2

We now allow you to set an expiry with a timedelta, and warn when you are using a value which the server will interpret as being in the past. Further disambiguating, we could allow either a timedelta or a datetime. Then, the user's intent is clear. Lets to that.

I believe we should still warn when the time is in the past - using the same 50 year stuff that is in there now. See couchbase/options.py, in timedelta_as_timestamp.

The difference here will be the > 50 years from 1/1/70 isn't suspicious. Instead, a datetime in the past should be warned (since it expires immediately and goes away).