Make jitter has a clear value range

[Xuanwo]

          > > Maybe a new function like `ConstantBuilder::unlimited` would be better for this?

Seems not a good idea to me. I plan to add a FnBuilder that accept a function which will return the next delay like the following:

let f = FnBuilder::new(|| Some(Durtion::from_secs(1)));

What do you think?

It should follow the principle of least surprise. User of library shouldn't look into the implementation to find out what function will do. I've spent some time finding out why it nearly instantly exits with such setup:

backon::FibonacciBuilder::default()
            .with_jitter()
            .with_min_delay(Duration::from_millis(5))
            .with_max_delay(Duration::from_millis(100))

Originally posted by @0xdeafbeef in #48 (comment)

[Xuanwo]

No actions to take so far.