harden HSTORE registration

harden HSTORE registration

* use the driver_connection when we register on the connection
* assert targets passed to register_hstore assert as boolean true;
  psycopg docs say "if None, register globally" but looking in the
  source it's actually registering globally if any false-evaluating
  object is passed.

Change-Id: Ie1fd7c96714b7fe76ef964501691fa48352be259
(cherry picked from commit 29569ccfde7247a7e0ed2afe43db53494da62fb2)