RetryNotify does not deal with operations that hang, even if a timeout is set

[MattCosturos]

I have 0 experience developing in go, so please be gentle if I misunderstand any of the inner workings of multi-threaded go development.
However to me, it appears that RetryNotify does not deal with operations that hang, even if a timeout is set.

RetryNotify uses RetryNotifyWithTimer uses doRetryNotify

doRetryNotify
Line 87 starts a for loop.
Line 88 calls the provided operation()
This is a blocking call. It waits for operation to return, and checks the return value.
Further down is a select block to wait for ctx.Done(), or the result of a timer.
But if operation() on line 88 blocks we would never get here?

I would imagine you need some code like the following inside the for loop. Again, I have 0 experience in go, this is based off what I've learned in the past 24 hours.

	errCh := make(chan error)
	resCh := make(chan res)

	go func() {
		res, err := operation()
		if err != nil {
			errCh <- err
			return
		}
		resCh <- res
	}()

	select {
	case <-ctx.Done():
		return ctx.Err()
	case err := <- errCh
		//do the error logic here
	case res := <- resCh
		return res, nil
	}

[MattCosturos]

Submitted PR #147 which keeps functionality the same (all existing test cases pass) and adds a test for a blocked operation