This PR adds a new field optional field `timestamp_location` that allows
the user to specify a timezone different than the default UTC for use in
the snapshot suffix.
I took @mjasnik 's PR https://github.com/zrepl/zrepl/pull/785 and
refactored+extended it as follows:
* move all formatting logic into its own package
* disallow `dense` and `human` with formats != UTC to protect users from
stupidity
* document behavior more clearly
* regression test for existing users
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
Christian Schwarz