Clean up postgres connection handling

Clean up postgres connection handling

* Remove non-connection magic case when no DB $user
  is given. This was removed from the base class.
* Use PGSQL_CONNECT_FORCE_NEW to let LoadBalancer
  handle connection reuse. This makes it work like
  the mysql classes.
* Make postgres connection error messages actually
  be useful by using the PHP error when possible.
  This makes it clear if the problem is authentication
  or something else and so on.

Change-Id: I3fd76c1e2db8d6008074f5347b201554579b549a