async support

[IGI-111]

I just came across this library and I definitely would want to use it but I can't because it doesn't support futures.

I know there are a couple of implementations as PRs already but I'm opening this to push for a decision now that the async ecosystem is probably mature enough.

[rbtcollins]

I think we should do something, because having to have two retry crates isn't super sensible; but we probably need to be smart about the runtime binding to sleep(); since we shouldn't block the thread, but -last I checked - core rust doesn't define a sleep async primitive.

[cbeck88]

Maybe can use some dependency injection here? So the caller can specify what sleep api to call, be it tokio::time::sleep or something else?

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Edit: I guess #28 is using dependency injection here

[jimmycuadra]

I'm personally not inclined to develop this crate further. I think async retries should be in a separate crate. But since I'm not very active in free software development anymore, I won't stop the other folks with repo/crate access if they want to build and maintain it.

[clarkmcc]

@jimmycuadra any chanced we could get #44 merged?