Replace `system::time::Duration` with `std::time::Duration`

[joriskleiber]

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This pull request replaces system::time::Duration with std::time::Duration.

I couldn't find any code that still uses negative duration, so I replaced it with it's std equivalent. The only place, where I'm not sure if its possible to use an negative duration, is the prior_validity property from the Authentication struct.

Should close #1170