[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/bin/cubicweb-ctl usr/bin/
usr/lib/python2*/*-packages/cubicweb/cwctl.py
../cubicweb-ctl.bash_completion etc/bash_completion.d/cubicweb-ctl