removeAndDeliver(buf, sender, loopback, cluster_name); // at most 1 thread will execute this at any given time
removeAndDeliver(buf, sender, loopback, null); // at most 1 thread will execute this at any given time
removeAndDeliver(buf, sender, loopback, cluster_name); // at most 1 thread will execute this at any given time
removeAndDeliver(buf, sender, loopback, null); // at most 1 thread will execute this at any given time