[entities] Optimize CWUser.properties.
This property gets called each time a user connects to a repository,
which means a lot.
This optimisation cuts its execution time by 5 according to my tests
usr/lib/PY_VERSION/*-packages/cubicweb/entities/
usr/lib/PY_VERSION/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/PY_VERSION/*-packages/cubicweb/*.py