[QOLDEV-490] use fast loading for resources that already have a data …

…dictionary

- Tabulator is good at type guessing but is slow. Once it has configured the column types, there's no need to use it every time

ckanext/xloader/jobs.py

@@ -212,7 +212,7 @@ def tabulator_load():
     logger.info("'use_type_guessing' mode is: %s",
                 use_type_guessing)
     try:
-        if use_type_guessing:
+        if use_type_guessing and not loader.datastore_resource_exists(resource['id']):
             tabulator_load()
         else:
             try: