startPipeline deadlocks when two xlinkouts have the same stream name

[jdavidberger]

After debugging for a while, it turns out that startPipeline will deadlock when you have two xlink nodes which share a streamname. While this configuration wouldn't make sense to run; it would be beneficial if this was validated and throws an exception instead of deadlocking.

[themarpe]

I agree, we'll add a check for this case