CloseNotify on Drop / explicit close

[ShadowJonathan]

#7 describes some wanted features with the following derivative text;

TCP has the concept of FIN packets, which let the client know that the connection they currently are using is dead and should be terminated. UDP has no equivalent. For this reason, restarting the server will take up to the connection timeout
value for clients to detect a dead connection. This can be shortened by modifying DTLS to send CloseNotify alerts when the UDP socket receives unrecognised data.

Tackling this from the other side, there should be a possibility to somehow send a "last gasp" CloseNotify on drop, or allow an explicit close function.

However, the design of #1 separates the transport layer with the record layer, so the connection object can be dropped separately than the socket, which doesn't make this much possible.