- Optimized the msg_list_item structure to make the buffer an

- Optimized the msg_list_item structure to make the buffer an
  integral part of the structure instead of pointer to a
  separately allocated buffer.
- Improved the code that was ensuring that no messages from the
  receiver could be merged into the middle of a partially written
  buffer from the generator.  The new code ensures that we never
  avoid reading the messages from the receiver (like we used to).
  This ensures that the generator will not hang when the receiver
  got a read error on the socket, sent us a message about it, but
  the socket in the generator never becomes writable for it to get
  a similar error (now we are assured of getting the receiver's
  note about their read error, and we know to shut things down).