Fix multiqueue leaking buffers and events when downstream or the queue are flushing. Make refcounting assumptions exp...

Original commit message from CVS:
* libs/gst/base/gstdataqueue.c:
* libs/gst/base/gstdataqueue.h:
* plugins/elements/gstmultiqueue.c: (gst_single_queue_push_one),
(gst_multi_queue_item_new), (gst_multi_queue_chain),
(gst_multi_queue_sink_event):
* tests/check/elements/multiqueue.c: (multiqueue_suite):
Fix multiqueue leaking buffers and events when downstream or the
queue are flushing. Make refcounting assumptions explicit and
document them (shouldn't break existing code that uses it other than
maybe leak miniobjects, but that already happens anyway). Add unit
test for the most common flushing case. Fixes #423700.