Cancelable parallel

[jelmervdl]

Fix for the non-quiting beviour of #116.

This adds a ThreadPool and a CancelableQueue.

The CancelableQueue combines a blocking queue with a stop signal. The problem is that the worker threads can only be interrupted when they're blocking on getting work from a queue, or putting results in a queue. This queue then has interruptible get() and put() methods.

The ThreadPool class is a better implementation of RaisingThread, where TheadPool.join() will behave more like a select() on sockets and raise an error as soon as any of the threads encountered an exception.

The general idea is that we start all threads as if everything will be allright, putting them in the thread pool. Then we wait for any of them to crash (or all of them to finish without exceptions). If any of them crashes, we call cancel() on the queues so that all the other threads also abort their loops. Then, because we're exiting the context of the ThreadPool, we'll wait for them to finish up nicely so that stuff like try ... finally still works as expected.

It is a contious decision that I don't care about exceptions in any of the other threads once I caught one in one of the threads in the threadpool. You can call join() multiple times until it exists without throwing, but in OpusCleaner just one error is already one too many or something.

[jelmervdl]

I'm waiting for CCmatrix en-ru to download so I can replicate and verify that this exact bug has now been fixed.