matroskamux: Delay stream-header until all sink pads have caps & don't time out in live mode if no timestamped next buffer is available
If we don't wait, an incomplete header might be generated due to a race between the _aggregate thread and the sink pad setcaps.
Fixes #3929 (closed)
The muxer can only advance the time if it has a timestamped buffer that can be output, otherwise it will just busy-wait and use up a lot of CPU.