internal: clean_up function has been added which clears all internal structs (event_message_queue, monitors).
message_queue: message_queue namespace has been added.
message_queue: event_message_queue has been moved from internal to message_queue.
message_queue: macros-like push functions have been moved to separate files and have been made inline.
message_queue: push_event_message and pop_event_message functions have been added.
tests: event_message_queue test has been edited due to recent changes.