Close server-side websocket connection after certain period of inactivity.

[asvetlov]

The only reliable way to make sure that socket is still alive is listening for ping message.

Server should terminate web socket if no incoming event was for timeout period.

By default the period should be None for keeping backward compatibility.

[jincreator]

If nobody is working on it, I'll take it.

[fafhrd91]

We have now receive_timeot and heartbeat, should be enough

[lock]

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