Improve Subnet delete performance

- don't re-fetch subnet object from DB
- network is not used in SubnetContext when deleting subnet

Above gives ~35% improvement

Change-Id: I34f850782092f771482a297ae1e68a63ffb027c1
(cherry picked from commit bdd50ffcde)

neutron/db/db_base_plugin_v2.py

@@ -1072,7 +1072,8 @@ class NeutronDbPluginV2(db_base_plugin_common.DbBasePluginCommon,
         with db_api.exc_to_retry(sql_exc.IntegrityError), \
                 db_api.CONTEXT_WRITER.using(context):
             registry.notify(resources.SUBNET, events.PRECOMMIT_DELETE,
-                            self, context=context, subnet_id=subnet.id)
+                            self, context=context, subnet_id=subnet.id,
+                            subnet_obj=subnet)
             subnet.delete()
             # Delete related ipam subnet manually,
             # since there is no FK relationship

neutron/plugins/ml2/plugin.py

@@ -1317,11 +1317,11 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
     @registry.receives(resources.SUBNET, [events.PRECOMMIT_DELETE], priority=0)
     def _subnet_delete_precommit_handler(self, rtype, event, trigger,
                                          context, subnet_id, **kwargs):
-        subnet_obj = self._get_subnet_object(context, subnet_id)
+        subnet_obj = (kwargs.get('subnet_obj') or
+                      self._get_subnet_object(context, subnet_id))
         subnet = self._make_subnet_dict(subnet_obj, context=context)
-        network = self.get_network(context, subnet['network_id'])
         mech_context = driver_context.SubnetContext(self, context,
-                                                    subnet, network)
+                                                    subnet, network=None)
         # TODO(kevinbenton): move this mech context into something like
         # a 'delete context' so it's not polluting the real context object
         setattr(context, '_mech_context', mech_context)