Re-worked the queue-runner

- The protocol between the queue-runner and the sender (in this case
  repr-pickle-sender) now includes using fd0 as a pipe back to the queue-runner
  in order to communicate performance metrics.

- The issue with the number of sub-processes creeping past the
  configured parallelism max has been fixed.

- In order for the queue-runner to deliver its own performance data,
  the relay on this system must now have a udp carbon listener in
  order to accept those metrics (this allows the relay to be turned
  off, but the sender to run without blocking or any clever code).