Memory leak in `Future<T>::after()`

The problem arises when one tries to associate an after() call to copied futures. The following test case is enough to reproduce the issue:

TEST(FutureTest, After3)
{
  auto policy = std::make_shared<int>(0);

  {
    auto generator = []() {
      return Future<Nothing>();
    };

    Future<Nothing> future = generator()
      .after(Milliseconds(1),
        [policy](const Future<Nothing>&) {
           return Nothing();
        });

    AWAIT_READY(future);
  }

  EXPECT_EQ(1, policy.use_count());
}

In the test, one would expect that there is only one active reference to policy, therefore the expectation EXPECT_EQ(1, policy.use_count()). However, if after is triggered more than once, each extra call adds one undeleted reference to policy.