[Godot] Make the reading process async

[CarlCochet]

Currently, the entire reading process is based on an infinite loop from which we only come out when receiving a full command. It would be nice if the engine could run normally (while keeping the physics process synced with steps) while the reading process happens in the background asynchronously.

For instance, right now if you use the debug camera, you can only move it when you receive a command, which is annoying.