Listener examples need to take all samples in the callback

[elBoberido]

Required information

The listener examples need to show that one needs to take all the samples of a subscriber, else there will be the illusion that samples are lost.

This can happen when a publisher produces more than one sample while the listener callback is executed. If the next callback execution then takes only one sample, the queue starts to pile up until eventually an overflow happens or the producer is blocked.

The examples need to show the correct usage of the listener.

See also #1541 (comment)

[elBoberido]

@mossmaurice can you check whether the discovery example also needs changes?