queue2: Refuse all serialized queries when posting buffering messages

When posting buffering messages there are no safe places or timing to avoid deadlocks.

Previously the code was trying to be "smart" by only forwarding serialized queries if the queue was empty ... but that could happen when queue2 hadn't yet posted a 100% buffering message. Meaning the pipeline might be paused and pushing a serialized query downstream might never complete.

Therefore let's completely disable forwarding of serialized queries when queue2 is used as a buffering element (meaning ALLOCATION and DRAIN queries).