[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.

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