Async framework needed a way to do repeating operations (think simplest
cases like AmendmentBlockHandler).
This PR implements the **absolute minimum**, barebones repeating
operations that
- Can't return any values (void)
- Do not take any arguments in the user-provided function
- Can't be scheduled (i.e. a delay before starting repeating)
- Can't be stopped from inside the user block of code (i.e. does not
have stop token or anything like that)
- Can be stopped through the RepeatedOperation's `abort()` function (but
not from the user-provided repeating function)
