queue: Ignore thresholds if a query is queued

The queue gets filled by the tail, so a query will always be the tail
object, not the head object. Also add a _peek_tail_struct() method to the
GstQueueArray to enable looking at the tail.

With unit test to prevent future regression.

https://bugzilla.gnome.org/show_bug.cgi?id=762875
4 files changed