Add a clean disconnection function to the client API

[nathhB]

There is currently no way to cleanly disconnect from the server on the client side: you can just stop the client and the server will detect the disconnection after a few seconds as packets will no longer be received.

The idea is to add a function to the client API that attempts to clean disconnection by sending a specific (library reserved) message (reliably) from the client to the server. Upon reception, the server should probably just close the connection.

On the client side it might be needed to add a "disconnecting" to probably handle the amount of time between sending the disconnection message and actually being disconnected.

[nathhB]

WIP: https://github.com/nathhB/nbnet/tree/feat/disconnection

[nathhB]

#11

Closed!