Enhancements from the last PR

[Marenz]

• Use shorter variant of the same code
• Clean up retry-tasks on shutdown

[llucax]

A couple more comments and a couple related thoughts:

1. Nitpick, but the class should also probably be called FailedDispatchesRetrier, now it is named as a function.

2. Is there any reason to make retrying optional? Is there any case you can think of where retrying is not desired? It makes the code more complicated and I don't think we really need it.

3. In the future I would like to move the retry module to the core lib so we can use it in more places, like here. If we do so, you can have a retry strategy that it is basically no retry, but the code will still be simple as the retry logic will not change.

4. Lately I was also thinking about this frequenz-floss/frequenz-sdk-python#1176, but it is harder to think about something generally useful for an arbitrary background service.