multiqueue: Fix high_time wakeup logic
multiqueue: Fix high_time wakeup logic

When calculating the high_time, cache the group value in each singlequeue.

This fixes the issue by which wake_up_next_non_linked() would use the global
high-time to decide whether to wake-up a waiting thread, instead of the group
one, resulting in those threads constantly spinning.

Tidy up a bit the waiting logic while we're at it.

With this patch, we go from 212% playing a 8 audio / 8 video file down to less
than 10% (most of it being the video decoding).

https://bugzilla.gnome.org/show_bug.cgi?id=770225

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer