[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)
Document: cubicweb-doc
Title: CubicWeb documentation
Author: Logilab
Abstract: Some base documentation for CubicWeb users and developpers
Section: Apps/Programming
Format: HTML
Index: /usr/share/doc/cubicweb-documentation/index.html
Files: /usr/share/doc/cubicweb-documentation/*.html