Store user groups and properties as session data * stop retrieving them systematically, only when need, * reimplement session synchronization hooks with some cleanups along the way, * cleanup call to set language: not needed from the base request nor from the server side, only for the web request (on the server side, language is necessary only for notification and such code should set it explicitly). There is still a XXX remaining about one can only "enter" a connection once and this is a problem in some cases. IMO, this restriction could be removed. Closes #13500113.

usr/lib/python2*/*-packages/cubicweb/devtools/
usr/lib/python2*/*-packages/cubicweb/skeleton/
usr/lib/python2*/*-packages/cubicweb/test
usr/lib/python2*/*-packages/cubicweb/dataimport/test
usr/lib/python2*/*-packages/cubicweb/entities/test
usr/lib/python2*/*-packages/cubicweb/ext/test
usr/lib/python2*/*-packages/cubicweb/server/test
usr/lib/python2*/*-packages/cubicweb/sobjects/test
usr/lib/python2*/*-packages/cubicweb/hooks/test
usr/lib/python2*/*-packages/cubicweb/web/test
usr/lib/python2*/*-packages/cubicweb/etwist/test