link: avoid multiple concurrent negotiations

When multiple links are created at the same time for the same port, we
get into a race where multiple links will try to set a format
asynchronously and eventually break the links. Avoid this by marking the
port as busy for as long as an async format or buffer is pending and
avoid starting new link negotiation when one of the ports is busy.

This problem was observed when ardour6 tries to link all device capture
ports to its single monitor port.