Wait for Eos message on the bus for signalling global end-of-stream

[sdroege]

Each appsink will signal eos individually and considering the first
one's eos as the global end-of-stream might cause the pipeline to be
shut down before all other appsinks are done processing their data.

The Eos message on the bus is only sent once all sinks are
end-of-stream.

Fixes #151

---

Untested but should fix the race condition. I've also added the line to send something to the mpsc sender because a few lines below a comment claims that we // Wait until we get an error or EOS., and only the error cases were covered.

[Manishearth]

This works locally, I can no longer reproduce the crash.