[buteo-syncfw] Don't allow plugins using the same client plugin to run at the same time.

Don't try to reuse processes for OOP client plugins.
We could be smarter here and play with the sync queue in order to allow the next syncs of different type to start if they exists, but requires some more checks in order to avoid loops and should be rare situation. I don't see this as critical, so prefer to keep it simple instead.