Refactor request-retry to be asynchronous up to the retry strategy

[lkuffo]

Is there any way to implement a custom retryStrategy which needs to be asynchronous?

Let's say, for example, my mustRetry option depends on some value that I have to query on a database.

[FGRibreau]

Hello!

Ahhh I was so young :) Indeed the whole API should be asynchronous (I did not make the mistake in the last libs), I will accept a PR (with tests) that will check if the yield value from the strategy is a promise and handle that accordingly :+1: