function OutBatch() {
  this.content = [];      // buf, flags, buf, flags, ...
  this.cbs = [];          // callbacks
  this.isClosed = false;  // true if the last message does not have SNDMORE in its flags, false otherwise
  this.next = null;       // next batch (for linked list of batches)
}
A batch consists of 1 or more message parts with their flags that need to be sent as one unit