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).

Part-of: <gstreamer/gstreamer!797>
36 jobs for !797 with backport-mr-796-into-1.18-queue2-buffering in 34 minutes and 1 second (queued for 5 seconds)
merge request