static CancellationScheduler finished(boolean server) {
        if (server) {
            return DefaultCancellationScheduler.serverFinishedCancellationScheduler;
        } else {
            return DefaultCancellationScheduler.clientFinishedCancellationScheduler;
        }
    }
A {@link CancellationScheduler} that has already completed.
core/src/main/java/com/linecorp/armeria/internal/common/CancellationScheduler.java