[web] Fix expiry of anonymous sessions (closes #4154479) Things like: try: foo except: bar else: baz doesn't work very well if you expect baz to run even in the exception case. Plus, session.cnx.check() is a dbapi remnant, so drop it and just look at session.mtime.

usr/bin/cubicweb-ctl usr/bin/
usr/lib/PY_VERSION/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl