Skip model validation when models are known good.

In some situations Django calls model validation when the models are
already known good. This is most visible in tests, which use flush
and loaddata commands. This resulted in around 10% overhead when
running tests under sqlite.

django/core/management/commands/syncdb.py

@@ -160,4 +160,5 @@ class Command(NoArgsCommand):
         # Load initial_data fixtures (unless that has been disabled)
         if load_initial_data:
             from django.core.management import call_command
-            call_command('loaddata', 'initial_data', verbosity=verbosity, database=db)
+            call_command('loaddata', 'initial_data', verbosity=verbosity,
+                         database=db, skip_validation=True)