[server/session] make commit/rollback obey ensure_cnx_set
Inside the ensure_cnx_set context manager, commit and rollback aren't
allowed to dispose of the cnxset.
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/