[server] Fix AttributeError in extid2eid
Session._tx was renamed in cb87e831c183, use _cnx instead.
#! /bin/sh -e#if [ "$1" = configure ]; then if which install-docs >/dev/null 2>&1; then install-docs -i /usr/share/doc-base/cubicweb-doc fifi#DEBHELPER#exit 0