Make backoff.duration available until the first next call.

[yowu]

This PR is attempt to fix the issue reported by issue #30.
The basic idea is to make sure duration call must be called after the first next call. This will make sure DelegateBackoff and ExponentialBackoff could be initialized properly based on the input.

[yowu]

@connor4312, any comments?

[connor4312]

I put in some changes to make the backoff factory separate from the backoff instance. I always prefer to try to use the type system to make it 'impossible' to write incorrect code or code that throws at runtime, doing it in this way enforces that duration() can't be accessed before the backoff is created in context the first time.

Let me know what you think.

[yowu]

The idea of "making the backoff factory separate from the backoff instance" is more clear! Good job!

[connor4312]

Cool. I'll get this merged. This is a breaking change, so it will be a major version bump