Consider setting up a thread pool

**Issue by [Dmitriy Morozov](https://bitbucket.com/mrzv)**
_Sunday Feb 08, 2015 at 18:44 GMT_

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Currently, we launch a new set of threads with every `foreach()` (and join them all at the end). It might make sense to create a pool of threads when `Master` is initialized, and then just reuse them during the `foreach()` computation.
