Bugfix: Retry mechanism doesn't respect the context

[SeBBBe]

This PR removes the Sleep func in the retry state and instead uses the following contruct to either wait the determined amount of time, or cancel if the context is canceled. I also added a test case that reproduces this scenario.

select {
case <-ctx.Done():
    s.Errs = []error{context.Canceled}
    return false
case <-time.After(sleepTime):
}