Graceful shutdown of event handlers

[Ben Smith]

Allow an event handler to be gracefully shutdown by its supervisor when supervised. By default a child process is shutdown by a supervisor by sending an exit signal with the :shutdown reason and allowing the process 5 seconds to terminate. This only applies when the child process is traping exits, otherwise the :shutdown signal will terminate the child process immediately.

Event handler GenServer processes do not currently trap exit signals so will terminated immediately. It would be useful to support graceful shutdowns.

• Supervisor shutdown values

[Ben Smith]

Closed by #431.