Deregister cancellation callbacks

[john4744]

Potentially resolves the problem described in #589.
We have a workflow where we use the same long-lived cancellation_token_source for lots of tasks. We then use when_all to schedule some work once those tasks complete. We were seeing continuous growth in memory usage, and also in the amount of time it would take to create new tasks and to deregister any cancellation callbacks that were associated with the cancellation_token.
Here is some repro code:

pplx::cancellation_token_source cts;
auto token = cts.get_token();
std::string msg;
auto start_time = std::chrono::high_resolution_clock::now();
auto end_time = std::chrono::high_resolution_clock::now();
std::array<pplx::task<void>, 100> tasks;
std::ofstream ofs("data.csv");
ofs << "construction time,deregister_callback time\n";
for (int i = 0; i < 1000; ++i)
{
  auto token_reg = token.register_callback([&]() {});

  start_time = std::chrono::high_resolution_clock::now();
  for (auto& task : tasks)
  {
    task = pplx::task<void>([&] {}, token);
  }
  end_time = std::chrono::high_resolution_clock::now();
  msg = std::to_string(std::chrono::duration_cast<std::chrono::microseconds>(end_time - start_time).count()) +","; // construction time

  pplx::when_all(std::begin(tasks), std::end(tasks));

  start_time = std::chrono::high_resolution_clock::now();
  token.deregister_callback(token_reg);
  end_time = std::chrono::high_resolution_clock::now();
  msg += std::to_string(std::chrono::duration_cast<std::chrono::microseconds>(end_time - start_time).count()) + "\n"; // deregister_callback

  ofs << msg;
}

Test Results On Master
Takes about 30s to execute. Memory usage starts around 2.4MB and climbs to around 13MB (using VS memory diagnostic tool).
The timing data gathered in the test is reflected in these charts:

Test Results On This Branch
Takes about 1s to execute. The VS memory diagnostic tool didn't have enough time to take a sample, so I increased the number of loop iterations to 100,000. The Memory usage starts around 2.2MB and stayed around there, ending at 2.4MB.
The timing data gathered in the test is reflected in these charts:

[msftclas]

All CLA requirements met.