Refactor traits and how they're used

[saviorand]

Currently traits are not used properly. E.g. actual server implementations (PythonServer and SysServer) do not inherit from the ServerTrait. This is because historically I've done things like return concrete implementations from a function that returns a trait, and this was removed from Mojo after a certain version. Need to rethink how traits are used and in which cases they are valuable, and refactor accordingly.