[dataimport] move cwmassive_constraint temporary table handling to the PGHelper class with consistent renaming and some rationalization to avoid doing things several times and have code slightly easier to grasp.

usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc