More extensive message system

[jwaa]

Is your feature request related to a problem? Please describe.
The current message system between agents works. However, there are some minor issues and good ideas for improving it. We need to think whether these warrant a (distant) future overhaul of this core functionality of MATRX.

Describe the solution you would like
Needs to be discussed based on actual encountered issues and ideas.

Describe alternatives you have considered
N/A

Additional context
Let's use this issue to structure the discussion around an overhaul of the message system. Here we can refer to smaller issues that touch or should be part of this overhaul.

Related issues
#201
#112
#254