[application] call req.set_session in application.main_handle_request
The Session handling chain is no more responsible for calling req.set_session.
It just returns a valid session and lets the caller link it to the Request.
This opens the way to explicitly creating and closing a connection/transaction
in ``application.main_handle_request``.
Related to #2503918
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/entities/ usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/ext/ usr/lib/PY_VERSION/site-packages/cubicweb
debian/tmp/usr/share/cubicweb/cubes/ usr/share/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/*.py usr/share/pyshared/cubicweb