Fixed #16250 -- Made test database creation support code in the PostgreSQL DB...

Fixed #16250 -- Made test database creation support code in the PostgreSQL DB backend compatible with psycopg2 2.4.2.

Implemented this by adding an internal hook for work that should be performed
before that point.

Also, regarding the `DatabaseCreation.set_autocommit()` method:
 * Stop using it for such tasks
 * Stop providing an implementation that tries to cover all the possible
   idioms a third party database backend DB-API 2 driver could need to activate
   autocommit. It is now left for third party backends to implement.

This can be backwards incompatible in the case of user applications that:
 * Had started using this method
 * Use a third a party database backend

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16520 bcc190cf-cafb-0310-a4f2-bffc1f526a37