Consider a constructor consuming `CompletionQueue` for `AsyncClient`

[coryan]

As @dbolduc reminded me, the design docs calls for no automated background threads. I think we should refactor the existing classes to something like:

class CompletionQueueServicePool {
 public:
  /// Create a thread pool of size @p size to service @p cq.
  CompletionQueueServicePool(CompletionQueue cq, std::size_t size);
};

And then the AsyncClient factory becomes:

AsyncClient MakeAsyncClient(CompletionQueue cq, Options options = {});