Introduce a warning timeout for MQTT topics

[gbeine]

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The feature allows to define a parameter 'timeout' for each topic. The timeout is specified in seconds.

Instead of notifying targets when a message arrives for a topic, mqttwarn will trigger the targets if no message is received on the topic for the specified period of time.

That's the first version of this feature, so any feedback is welcome.

[amotl]

Hi Gerrit,

apologies for the late reply. That's a nice liveness sensor you are contributing here. I didn't have a look at the details yet, but if @jpmens or @sumnerboy12 don't have any objections about adding that as a new kind of feature, let's try to bring it into mainline.

While your implementation looks pretty solid, I may have a few suggestions about design and implementation details as we go.

With kind regards, Andreas.

[gbeine]

@amotl Thank you - and no problem, we're all busy folks ;-)

I'd welcome any feedback as I'm not sure if I've done the implementation in the most effective way. All I can say is, that it runs on some Raspberry Pi 2 and 4 machines with about 70 to 150 topics stable for months.