More --timeout improvements, especially for the receiving side:

- The receiver now sends keep-alive messages to the generator
  when it is actively doing work and hasn't sent anything
  recently.  This ensures that the generator won't timeout
  if the receiver is working hard.
- The perform_io() code has improved keep-alive participation.
- Allow the sender to send some keep-alive messages, which
  ensures that if it is in a lull, it can probe the socket.