Implement RFC 50: OGR field subtypes. Impact core, OGR SQL, swig bindings, CSV, FileGDB, GeoJSON, GML, GPKG, OpenFileGDB, PG, PGDump, SQLite, VRT

From: Even Rouault <even.rouault@mines-paris.org>

git-svn-id: https://svn.osgeo.org/gdal/trunk@28008 f0d54148-0727-0410-94bb-9a71ac55c965
GeoJSON: implement Date/Time/DateTime field type detection

git-svn-id: https://svn.osgeo.org/gdal/trunk@28009 f0d54148-0727-0410-94bb-9a71ac55c965
Fix warning in non-debug build

git-svn-id: https://svn.osgeo.org/gdal/trunk@28010 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx