Listeners not started after fork
When trytond process is forked (for example by µwsgi), new listeners for the Cache and the Bus are not started if they were started on the parent process. This is because we use the thread object stored in a dictionary as detector to start or not. But on the forked process we still have the value from the parent. This is the same problem as #8281 (closed) for database connection so I think we should solve the same way by using the PID when storing such values.