Create actual authentication between the clients and server

[lawndoc]

Currently, the server knows a connection is from a permitted client based on a port-specific "knock" phrase sent upon connection. Adding an actual authentication method to ensure only permitted clients can connect would be more secure.

[lawndoc]

As of a5d3b47 clients submit an arbitrary connection key and are queued until they time out or can be bridged with their counterpart client that has a matching connection key. As of now, this process is unencrypted. Going to leave this issue open in order to encrypt the connection keys being sent.

[lawndoc]

As of 5ae24b3 the connection key must start with the prefix "#!ConnectionKey_" without the quotes. This drops all client connections that don't contain a valid prefix in the connection key message. This process is currently still unencrypted.

[lawndoc]

Certificate-based authentication is probably a better way to go here...