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.

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port:4:export

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port:4:vaportight

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port:4:import/export

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FIXME: history is wrong here

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port:1:1:-:eggs:import

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port:1:2:+:eggs:vaportight

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port:1:3:+:eggs:import/export

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port:0:2:+:spam:export

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port:0:1:+:spam:import

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port:4:import/export