postgres provider update

- track open cursors and commit transaction whenever no open cursors are left
- close cursor after immediately after feature retrieval (releases locks earlier)
- wrap postgres calls in Conn class
- introduce Result class to handle PQclear mostly transparently (fixes minor leaks)
- remove executeDbCommand method (duplicate of PQexecNR)
- introduce support for estimate_extents (fixes #582)
- introduce support for enum types (fixes #1210)



git-svn-id: http://svn.osgeo.org/qgis/trunk/qgis@9183 c8812cc2-4d05-0410-92ff-de0c093fc19c

Author: jef