Backoff constant should be configurable

[dalemyers]

Currently, if you use @delay_retry then the exponent is the constant 10. This means 5 repeats go from 0.05 -> 0.5 -> 5 -> 50 -> 500.

If for example, you have an API quota which has 60 second windows, you can't repeat 4 times as you would only cover 55.55 seconds. Instead, you need to go to the next one, which is over 8 minutes long. Being able to set a much smaller exponent would make this infinitely more useful. e.g. if 2 is used:

0.05 -> 0.1 -> 0.2 -> 0.4 -> 0.8 -> 1.6 -> 3.2 -> 6.4 -> 12.8 -> 25.6 -> 51.2

This way we only "waste" 42 seconds. Of course, every API and everyone's usage is different, so there many be many even better reasons to make this configurable.