flags: use time.Duration in duration flags

[jsign]

We currently have some arbitrary magnitude durations seconds/minutes as Int64 in flag/envs

Switch to time.Duration since it's much clearer and the user can specify them with 10m or 10h. This avoids confusion about the right magnitude: seconds or minutes?

This is a breaking change, since the operators should fix whatever flag/env that they might be overridden, so it would be good to batch into some planned major release.