Upgrade to netty 4.0.21

Open Task:
  src/riemann/transport.clj

The shared-event-executor is replacing the old thread pool (because it was
deprecated).
But there might be a better implementation available which gives you a similar
degree of control.

This should probably be addressed before the next release of riemann.

The current value is set to 100 (100 available threads) using the
DefaultEventExecutorGroup.