Adding an initial implementation for the unbuffered channel

[abhinavarora]

@kavyasrinet and I worked together on designing this initial code for Unbuffered Channels

[kavyasrinet]

As of now, the code proposed in this PR builds and compiles. We are working on coming up with a basic test case to test the functionality before we put it up for review.

[chengduoZH]

To ensure that UnbufferedChannel works well, I think that we should call Send and Receive many times.
e.g.

  size_t sum_send = 0;
  std::thread t([&]() {
    for (size_t i = 1; i <= 3; ++i) {
      ch->Send(&i);
      sum_send += i;
    }
  });
  size_t sum_rece = 0;
  size_t recv;
  for (size_t i = i; i <= 3; ++i) {
    EXPECT_EQ(sum_send, sum_rece);
    ch-> Receive(&recv);
    EXPECT_EQ(recv, i);
    sum_rece += i;
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
  }

[kavyasrinet]

Thanks so much for the feedback! We will be definitely adding more test cases to this file like the one you mentioned here.

[abhinavarora]

@chengduoZH Thank you for the feedback. Fixed it!

[abhinavarora]

This PR only contains a simple unit test. We will add more unit tests after the PR is merged.

[chengduoZH]

LGTM

[wangkuiyi]

It seems that this PR is in its very early stage -- we need many more unit tests to make sure that the implementation works.

[wangkuiyi]

Please make sure that the destructor calls CoseChannel if necessary, so that users can do delete without calling CloseChannel in prior.

[kavyasrinet]

Yes, we will be adding more unit tests in successive PRs as well.