The support of multiple producers

[Yanjiachan]

If multiple producers are employed, it could lead to a blockage at the location of the following code:

https://github.com/Squadrick/shadesmar/blob/master/include/shadesmar/pubsub/topic.h#L136

Below is my test code

struct stu {
    int a = 0;
    char b[10] = {0};
};

int test() {
  auto sp_pub = make_shared<Publisher>("test_shared_topic");
  if (!sp_pub) {
      return -1;
  }
  int i = 0;
  while(true) {
      struct stu s;
      s.a = i++;
      string str = to_string(i);
      strncpy(s.b, str.c_str(), str.length());
      bool ret = sp_pub->publish(reinterpret_cast<void*>(&s), sizeof(stu));
      if (!ret) {
          cout << "publish error" << endl;
      }
      this_thread::sleep_for(chrono::milliseconds(1));
  }
  return 0;
}

I spawn multiple processes to execute the above code.