queue: Add timeout for queries on leaky queue

The goal of a leaky queue is not block if downstream is blocked, but queries wait forever, so we'd rather drop the query after a timeout if it doesn't get answered in a reasonable time.

This is needed by gst-plugins-bad!2363 (merged), because webrtcbn puts a pad block after a leaky queue. Except that it's all useless if the thread gets blocked in an allocation query.