Document best practice for WebSocket keep alives

[andrewbonney]

TCP Keep Alives and WebSocket Ping/Pong are optional for clients/servers to implement. Whilst clean disconnections will be gracefully managed by WebSocket clients and servers, unexpected connection interruptions may not be detected on the client or server side until the next message is sent. This presents the greatest issue for a client as the Query API WebSocket messaging is only mandated as unidirectional from server to client. Best practice should be documented in order to avoid situations where a client fails to observe a loss of connection and becomes out of sync with the registry.

[garethsb]

This could go in a future AMWA INFO-xxx implementation guide?