PG: use ST_Estimated_Extent() on table layers if GetExtent() is called with force = 0 (derived from patch by gabrimonfa, #5427)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27106 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx