Clean up protocol date handling substantially

In RFC822.Date, don't use duplicate time_t and GLib.DateTime properties -- just use the latter, fix formatting of half-hour time zones, fix where the that class refers to ISO 8601 instead of RFC822 strings since that is just plain wrong, and finally when parsing an RFC 822 string, take note of the timezone offset and store that in the DateTime object, so it is round-tripped correctly.

Stop passing time_t around everywhere else, just use the UNIX time from the DateTimes we store for protocol objects anyway, so the time zone is obvious.

Add unit tests.