Call transport.serve with create_task instead of gather

With gather [e.g.](https://github.com/DiamondLightSource/FastCS/blob/main/src/fastcs/control_system.py#L102) the transport can block the application from shutting down. With create_task ([e.g.](https://github.com/DiamondLightSource/FastCS/blob/main/src/fastcs/control_system.py#L59)) they can't. Only the interactive shell should block the application so that it shuts down when that is stopped.

Currently pva does block, whereas ca does not so it stops cleanly.