[web/httpcache] send weak ETags There's no way we can provide a strong validator in general (especially considering how ETags are generated by the current EtagHTTPCacheManager and EntityHTTPCacheManager). So mark our entity-tags as weak.

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