Refactor message_queue.py

### Problem or idea
- get rid of global variables
- use object oriented approach
- optimize branching
- add tests

### Solution or next step
Implement the changes listed above.

Start by reading message_queue.py to identify its global variables, branching logic, and existing test coverage. Refactor the message queue toward an object-oriented design, simplify the branching, and add tests that cover the changed behavior; done means the listed changes are implemented and tested.

Written by the indexing model from the issue text.