Rewrite and modernise application code

[Jacalz]

Description:

This is a work in progress PR for a big rewrite of the application code. Most of the project structure and code dates back to the first versions and there is a lot of crust that has been maintained since then. The idea here is to rewrite and refactor to make the code easier to maintain, fix race conditions and just generally modernise the application. As part of this, support for multiple sends and receives at once will be removed as it adds more complexity than what it is useful.

Fixes #98, #143, #144, #145

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.