PG: use COPY mode by default (unless PG_USE_COPY is set to NO) when inserting features in a newly create table (#5460)

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27308 f0d54148-0727-0410-94bb-9a71ac55c965
test_ogrsf: adjustment in TestTransactions() to make it pass with PostgreSQL and PG_USE_COPY=YES

git-svn-id: https://svn.osgeo.org/gdal/trunk/gdal@27309 f0d54148-0727-0410-94bb-9a71ac55c965