systemclock: Add gst_system_clock_set_default

Used for setting the default system clock that is obtained through
gst_system_clock_obtain(), which is sometimes needed for unit
testing.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=711269
queue: Don't use gst_buffer_get_size() when possible

Makes qst_queue_locked_dequeue 20% faster