Originally, I had a patch that would replace all usages of
time.Duration in package config with the new config.Duration
types, but:
1. these are all timeouts/retry intervals that have default values.
Most users don't touch them, and if they do, they don't need
day or week units.
2. go-yaml's error reporting for yaml.Unmarshaler is inferior to
built-in types (line numbers are missing, so the error would not have
sufficient context)
fixes https://github.com/zrepl/zrepl/issues/486
