Clean up date and timezone handling. We used to pass changelog dates around as a "unixtime timezone" string containing a pair of encoded ints. Now, they get passed around as a (unixtime, timezone) tuple of numbers, which makes much more sense.

port:4:export
port:4:vaportight
port:4:import/export
FIXME: history is wrong here
port:1:1:-:eggs:import
port:1:2:+:eggs:vaportight
port:1:3:+:eggs:import/export
port:0:2:+:spam:export
port:0:1:+:spam:import
port:4:import/export