Specify target thread for MPWURLFetchStream

[mkarp]

MPWURLFetchStream would always call its targets (including the error target) on whichever threads the underlaying NSURLSession is running on.

This caused thread locking issues in the coalescing while-loop in -executeRequest: method: if a request is scheduled on the same thread that NSURLSession is running on, this thread would sleep, and so the NSURLSession would not be able to complete its request, and so the while-loop will run for a significant time (I've seen up to 25 seconds).

This pull request offers a solution to this problem.

[mpw]

That's a good idea, three notes:

1. There's already an NSOperationQueue

I don't remember exactly, but I think that one of the last changes I made was to initialize the NBSURLSession in XOBackend, and this NSURLSession is initialized with an NSOperationQueue. My understanding of this parameter to the NSURLSession is that the task-completion blocks are executed on this queue. So the subsequent processing should already be concurrent. If it's not concurrent enough, it should be easy to increase the concurrency parameter in where the NSOperationQueue is initialized (I think in XOBackend).

2. MPWThreadSwitchStream

If the NSOperationQueue is not sufficient or appropriate, I would simple add an MPWThreadSwitchStream when constructing the pipes. The idea of filters is that they do one thing well.

3. HOM

If neither the ThreadSwitchStream nor the NSOperationQueue suffice, I'd at least use the HOM:

[[target onThread:self.targetThread] writeObject:processed];

But that would be last resort.