title: c++ Grpc::CompletionQueue construction speed is around 14k?
body: <p>In async-grpc,I try to create a new grpc::completionqueue with every connection.
  Then,i find a problem.The construction speed of cq does not seem to be fast.</p>
  
  <pre><code>//It test with folly::benchmark
  BENCHMARK(CompletionQueue){
      grpc::CompletionQueue cq;
  }
  
  BENCHMARK(folly_mpmcqueue){
      folly::MPMCQueue&lt;int&gt; cq;
  }
  
  BENCHMARK(std_queue){
      std::queue&lt;int&gt; cq;
  }
  </code></pre>
  
  <p>the result is:</p>
  
  <pre><code>============================================================================
  test.cpp                                        relative  time/iter  iters/s
  ============================================================================
  CompletionQueue                                             63.99us   15.63K
  folly_mpmcqueue                                             67.12ns   14.90M
  std_queue                                                  220.97ns    4.53M
  ============================================================================
  </code></pre>
  
  <p>Cq does not seem to be free to construct and destruct.
  So should I reuse them instead of creating a new one for each connection?</p>