Don't directly access the in-memory DB for NB_Global row

Directly accessing the in-memory OVSDB via 'tables' outside of a
Command object isn't safe. For example, if we lose connection to
the OVSDB and reconnect, the in-memory copy will be replaced by
the Connection thread. If we get a call to get_agents() that is
handled by the main thread and we look up the value from there,
the value we expect may be missing.

The dependent patch in ovsdbapp adds some locking to help with
this some API commands like lookup() that are heavily used and
with ReadOnlyCommands which also aren't processed like normal
Command objects inside the connection thread.

There will be many more changes in the ml2/ovn code similar to
this one, as we directly access the in-memory copy a lot.

Depends-On: https://review.opendev.org/#/c/745746/
Change-Id: I328391c88af1ef2f6b1af3787c63d8591fb317b5
Closes-Bug: #1888878

neutron/plugins/ml2/drivers/ovn/mech_driver/ovsdb/impl_idl_ovn.py

@@ -166,7 +166,8 @@ class OvsdbNbOvnIdl(nb_impl_idl.OvnNbApiIdlImpl, Backend):
 
     @property
     def nb_global(self):
-        return next(iter(self.tables['NB_Global'].rows.values()))
+        return next(iter(self.db_list_rows('NB_Global').execute(
+            check_error=True)))
 
     def create_transaction(self, check_error=False, log_errors=True,
                            bump_nb_cfg=False):