Retry on result (not just on exception)

[Ladicek]

Sometimes, it is necessary to retry on a non-exceptional result (such as null or Optional.empty()). This should be fairly simple to implement, API design is the most problematic concern.

Programmatic API: this one is fairly simple, add a method whenResult(Predicate<T>) to the RetryBuilder. Possibly also rename when() to whenException() to better highlight what's going on. It is not obvious what to do if whenResult() is called and when[Exception]() isn't (and possibly retryOn() / abortOn() is).

Declarative API: we cannot do anything else but add a 2nd annotation. @RetryOnResult seems like a good name. It is not obvious how to define the predicate. Probably start small and add a single member Class<Predicate<Object>> value().