We have various constants to try and avoid mistyping of durations, e.g.
`ONE_HOUR_SECONDS * MILLISECONDS_PER_SECOND`, however this can get a
little verbose and doesn't help with typing.
Instead, let's move towards a dedicated `Duration` class (basically a
[`timedelta`](https://docs.python.org/3/library/datetime.html#timedelta-objects)
with helper methods).
This PR introduces the new types and converts all usages of the existing
constants with it. Future PRs may work to move the clock methods to also
use it (e.g. `call_later` and `looping_call`).
Reviewable commit-by-commit.
b74c29f694