cubicweb #736039 win32: stopping the cw service keeps references in Pyro NS [validation pending]

When running as a Windows service, it is currently not possible to stop and restart the service without stopping and restaring the pyro ns service in between as the instance is not deregistered when the CW service stops (causing the restart to fail at the pyro ns registration step).

priorityimportant
typebug
appeared in<not specified>
done in3.7.3
load0.200
load left0.000
closed by#5088:891cdb7d8cbb