[repo] ensure we don't restart a task while the repo is shutting down (closes #1942736) we need a safety belt in cw itself again looping task which use 'try / except' or doesn't need to access the db (which will trigger ShuttingDown exception)

usr/lib/python2.4/site-packages/cubicweb/
etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances