python: idl: Avoid sending transactions when the DB is not synced up.

This ports the C IDL change f50714b to the Python IDL:

Until now the code here would happily try to send transactions to the
database server even if the database connection was not in the correct
state.  In some cases this could lead to strange behavior, such as sending
a database transaction for a database that the IDL had just learned did not
exist on the server.

Signed-off-by: Terry Wilson <twilson@redhat.com>
Acked-by: Dumitru Ceara <dceara@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

python/ovs/db/idl.py

@@ -1505,6 +1505,11 @@ class Transaction(object):
         if self != self.idl.txn:
             return self._status
 
+        if self.idl.state != Idl.IDL_S_MONITORING:
+            self._status = Transaction.TRY_AGAIN
+            self.__disassemble()
+            return self._status
+
         # If we need a lock but don't have it, give up quickly.
         if self.idl.lock_name and not self.idl.has_lock:
             self._status = Transaction.NOT_LOCKED