So far, we used Connection.destroy, which destroys a connection without
sending out a CONNECTION_CLOSE frame. This is useful (for example) when
receiving a stateless reset, but it's not what we want when the server
refuses an incoming connection. In this case, we want to send out a
packet with a CONNECTION_CLOSE frame to inform the client that the
connection attempt is being rejected.
There's no need to have a dedicated shutdown method, as the use case
(shutting down an outgoing connection attempt on context cancellation)
can be achieved by using Connection.destroy.
