Allow and track multiple publisher to same topic

[eliasdc]

This PR adds the ability to have nodes publishing to the same topic to the benchmark application.

This adds a node_name attribute to the header message to keep track of the source of the message. A tracking number is saved for each unique node_name. This fixes an issue where the tracking_number_counter received the same number multiple times and the calculation of the stats failed because of it.

Size and frequency is saved in a Stat object to allow different sizes and frequencies with multiple publishers.

[alsora]

@eliasdc I want to make sure that the application performance are not affected by the change, should be able to get results tomorrow.

[alsora]

I compared your branch and master on a RaspberryPi 4 using the 20 nodes topology "montblanc". master CPU usage is steady at 8.3%, while your branch CPU usage is steady at 11%.

I prefer to not merge the change as it is. Next week will look more into the PR and see if we can optimize it somehow.