Relax strict ordering of client messages processing re: procedures

[gefjon]

As of writing, messages from a single WS client are always processed and responded to strictly in-order. This is almost correct, but should be loosened slightly for procedures, as they may be long running. CallProcedure requests should not block the processing of subsequent messages by the same client.

[joshua-spacetime]

Fixed by #4962 and #4973.