plugins/elements/gstmultiqueue.*: Take the multiqueue lock when updating the fill...

plugins/elements/gstmultiqueue.*: Take the multiqueue lock when updating the fill level so we don't get confused.

Original commit message from CVS:
* plugins/elements/gstmultiqueue.c: (gst_multi_queue_init),
(gst_single_queue_flush), (apply_segment), (apply_buffer),
(gst_single_queue_push_one), (gst_multi_queue_loop),
(gst_multi_queue_sink_activate_push), (gst_multi_queue_sink_event),
(gst_multi_queue_src_activate_push), (wake_up_next_non_linked),
(compute_high_id), (gst_single_queue_new):
* plugins/elements/gstmultiqueue.h:
Take the multiqueue lock when updating the fill level so we don't get
confused.
After applying a buffer or event on the src pad segment, make sure to
call gst_data_queue_limits_changed() to get the data queue to unblock
and check the filled state again.
Rework the not-linked pad handling so the logic is that not-linked
pads can push as fast as they like, but only so they never get
ahead of any linked pads.
* tests/check/elements/multiqueue.c: (mq_sinkpad_to_srcpad),
(mq_dummypad_getcaps), (mq_dummypad_chain), (mq_dummypad_event),
(run_output_order_test), (GST_START_TEST), (multiqueue_suite):
Add a test to check that not-linked pads always stay behind
linked pads.