[repo test] Avoid hangs in zmq repo unit test
- interaction through the zmq event loop from a different thread is only
allowed through add_callback
- notify the background thread when the client dies with an exception so it
doesn't spin forever
doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc