Graceful shutdown

[Kay-Conte]

This is the tracking issue for graceful shutdown of the server.

Target interface:

// Some endpoint handler
fn shutdown(control: Control) {
  control.shutdown();
}

Checklist:

• Potentially add shutdown to Connection
• Implement a method of globally shutting down the server. Likely a broadcast channel indicating to the client acceptor that no more clients should be accepted.
• Create a Resolveable handle to send the message from.

This should wait until #21 as it should allow for better ergonomics.