Fix: After the cursor leaves Thumbs, ZMS must always be stopped, even in case of network problems.

[IgorA100]

For all Thumbs we will assign the attribute "data-monitor-url" so that we can send ZMS the CMD_QUIT command without using MonitorStream.
We should not rely only on SIGPIPE, we must terminate ZMS correctly in any case.

Closed #4418

[connortechnology]

That isn't going to do it.

We need a keepalive. The zms cgi needs a keepalive parameter, so that if it doesn't get a status update request after so many seconds, it dies. We can't depend on CMD_QUIT.

[IgorA100]

That isn't going to do it.

Why? It works fine for me! I'm debugging JS now.

The zms cgi needs a keepalive parameter

Yeah, I agree. That would be a great option. But I don't know how to do it :(
If you did this in C++, I would arrange for a command to be sent to maintain the connection.

We can't depend on CMD_QUIT.

Yeah, but it's better than relying on SIGPIPE alone

[IgorA100]

This PR works!
But of course it is better to use keepalive for ZMS

[IgorA100]

@connortechnology
In this PR we add a attribute to the IMG so that we can send a command to the server where the monitor is served. This attribute will also be needed if you implement keepalive for ZMS. If keepalive is implemented for ZMS, then a very small change will need to be made to skin.js.
This PR works, but I certainly like the implementation of keepalive for ZMS more, but I can't do it in C++