Method checkin

async_simple/uthread/test/UthreadTest.cpp:149–151

    bool currentThreadInExecutor() const override { return true; }
    Context checkout() override { return NULLCTX; }
    bool checkin(Func func, Context ctx, ScheduleOptions opts) override {
        return schedule(std::move(func));
    }
};

// reschedule uthread to two different executor is not thread-safe