Improvements to the Retry Layer

[itsyaasir]

Hi, I have seen that in the new tower v0.5 they finally added the Backoff/ExponentialBackoff policy which can be used with the retry layer.

It will be great if this library will be able to integrate this.

Thanks

[geofmureithi]

Technically we have first class support for anything from tower.
That said, we need a robust solution that integrates well with different backends as the current approach handles retries in memory.