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.
This is more consistent with both the RFC and the rest of the API. For
example, the option in the Config is already name EnableDatagrams, and
the property in the ConnectionState is named SupportsDatagrams.
