[hook] remove assumption about entity cache vs cw_edited
We were assuming that if an entity is in the connection's cache, it has
a cw_edited attribute. This doesn't seem to always be the case. At
least in cubicweb 3.21, merging the web and repo-side caches makes this
more likely, but Denis says he's also seen this on 3.20.
../../doc/book usr/share/doc/cubicweb-documentation
../../doc/html usr/share/doc/cubicweb-documentation
../../debian/cubicweb-doc usr/share/doc-base/cubicweb-doc