Mark common postgresql types as non-enumerable upon load

This fixes a big performance issue when loading projects with PostgreSQL layers.
In the last stage of project loading, code in QgisApp would run code
to resolve broken layer dependencies, which in turn tried to find out best
edit widget for all columns of all layers, and the enumeration widget tried
to figure out all enum values, doing two SQL calls for each column for each layer.

We know what are types of columns when loading a layer, so enumValues() can leave
early without doing the excessive queries when it's not necessary.

Testing on a real world QGIS project with 300+ layers (but not all PostgreSQL):
- before: ~35 minutes to load / ~8K PostgreSQL queries
- after: ~10 minutes to load / ~2K PostgreSQL queries

src/providers/postgres/qgspostgresprovider.cpp

@@ -1388,6 +1388,11 @@ bool QgsPostgresProvider::loadFields()
 
     mIdentityFields.insert( mAttributeFields.size(), identityMap[tableoid][attnum][0].toLatin1() );
     mAttributeFields.append( newField );
+
+    // if we know for sure that this field is not enumerated type or a domain type, let's
+    // mark it here, so that enumValues() does not need to check it again (for types like int, text, ...)
+    if ( fieldTType != QLatin1String( "e" ) && !isDomain )
+      mShared->setFieldSupportsEnumValues( fields.count() - 1, false );
   }
 
   setEditorWidgets();