Improve message subsystem codebase

[LeeSmet]

There is quite some room for improvement here. One example: when an acknowledgement is received for a packet, we don't immediately send a new one but rather wait for the 1 second timer. Also iirc we only send 1 packet at a time. This can be improved by sending multiple at once. Other improvements may also apply

[xmonader]

Is that still going to be part of this release? @LeeSmet @maximevanhees that's already 2month ago

[LeeSmet]

This is on the backburner now since priorities changed