Remove continuation feature

[trask]

Continuations automatically keep the local root span open until all nested spans are finished, e.g. the duration of the trace below would be ~1 second, instead of the ~1 millisecond that it takes to return from the method.

@Trace
public Future<String> execute() {
    activeScope().setAsyncPropagation(true); // enables continuation support for this trace
    return executor.submit(() -> {
        Thread.sleep(1000);
        return "Hello!";
    });
}

It's a nice feature, but also brings significant complexity, so I think it's worth removing.

Also, there's no equivalent OpenTelemetry API feature, so this would require a special agent API, and then instrumentation that relied on this would further diverge from manual (contrib) instrumentation.