Function cancellation

src/test/common/test_ceph_timer.cc:119–137

template<typename TC>
void cancellation()
{
  ceph::timer<TC> timer;
  {
    std::promise<void> p;
    auto f = p.get_future();
    auto e = timer.add_event(100s, Destructo(std::move(p)));
    EXPECT_TRUE(timer.cancel_event(e));
  }
  {
    std::promise<void> p;
    auto f = p.get_future();
    auto e = timer.add_event(1s, [p = std::move(p)]() mutable {
                                   p.set_value();
                                 });
    f.get();
    EXPECT_FALSE(timer.cancel_event(e));
  }
}

template<typename TC>
void tick(ceph::timer<TC>* t,