Make all internal functions of S2Connection private by prefixing an underscore

Functions e.g. https://github.com/flexiblepower/s2-python/blob/main/src/s2python/s2_connection.py#L540 `async def send_msg_and_await_reception_status_async` are not prefixed by an underscore while they are meant to be private. This is causing some misuse and issues for users of the library.