If a client doesn't send a connection message, it may appear to be connected

[mdwaud]

Expected Behavior
A client that has not completed the connection request / response handshake does not show as connected in Sink.Server.connected_clients

Observed Behavior
Really old clients that needed to be updated and weren't sending connection requests showed up as connected.

This is because the registry doesn't differentiate between clients / processes with open TCP connections and those which have completed the connection process.