Tech debt around duration config

[sharnoff]

A few things here:

1. Our config files have durations for various things, and they're largely defined as "integer number of seconds"
2. We have a bunch of ad-hoc duration constants with FIXMEs to make them configurable
3. We have a dedicated type for picking random durations in a range (util.TimeRange), but we're not using that outside of util.Watch.

It'd be good to resolve some of these, ideally

• [ ] Using util.TimeRange in the configs; and
• [ ] Determining which duration constants should be made configurable (and which should be left as-is)