Add jitter

[lvh]

Right now, we have deterministic exponential backoff. This means users of banach potentially are exposed to stampeding herd problems.

We could do this in exponentially, or make it composable.

A composable API would look like [jitter-params strategy] -> strategy. If we use with-clock with some state, we can figure out what delays another strategy imposed and instead apply jitter and then run them with a real clock (unless if course it's composable turtles all the way down, life is a simulation, and all attempts at computer are intrinsically futile). This is not super great, but we didn't make up that global mutable state that is the reactoclock.

The benefit of a composable API is that it's reusable. The default, especially for helper fns, should absolutely include jitter.