splitmux: Rewrite buffer collection and scheduling
Majorly change the way that splitmuxsink collects incoming data and sends it to the output, so that it makes all decisions about when / where to split files on the input side. Use separate queues for each stream, so they can be grown individually and kept as small as possible. This removes raciness I observed where sometimes some data would end up put in a different output file over multiple runs with the same input. Also fixes hangs with input queues getting full and causing muxing to stall out.
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