multiqueue: Fix high_time computation

multiqueue: Fix high_time computation

* Avoid the computation completely if we know we don't need it (not in
  sync time mode)
* Make sure we don't override highest time with GST_CLOCK_TIME_NONE on
  unlinked pads
* Ensure the high_time gets properly updated if all pads are not linked
* Fix the comparision in the loop whether the target high time is the same
  as the current time
* Split wake_up_next_non_linked method to avoid useless calculation

https://bugzilla.gnome.org/show_bug.cgi?id=757353