max randomization factor can be 1, refactor randomization function

[jdcaperon]

TLDR: Nice to have but not important, this could have impacts on code that expects values returned from an interval function to be greater than zero.

This PR proposes two changes:

1. Allow randomization factor to be 1.0.
2. Refactor randomize() in IntervalFunction to remove +1 and to specify its contract.

Randomization factor to be 1.0

Largely cosmetic, however it does have the consequence of allowing interval functions to now return 0. I don't know enough about the rest of the codebase outside of Retry to know if this will have any negative consequences. If this is accepted I would also propose a new constructor ofExponentialFullRandomBackoff that defaults to a value of 1.0 for randomization. Full random or full jitter is generally a desirable characteristic for backoff functions: https://aws.amazon.com/blogs/architecture/exponential-backoff-and-jitter/

Refactor randomize()

Randomize currently includes a + 1 which seems out of place. This +1 has no effect on large intervals since it extends the range by an insignificant amount and when the interval is 0 it forces the randomize function to produce values between 0 and 1.

My proposal is to refactor randomize so that it produces values between 0.0 <= x < 2 * current.