Only import allocate_lock from thread in session.py

darcs-hash:20090130164730-58103-bed6c60d59afe4a2331678c137a4b415edd28ff4
Fix for Session.new() method in locating Invitation object

darcs-hash:20090130164752-58103-76bb6d31225337445b991d2f0bdd251098eef774