- QThread instead of std::thread for better integration with Qt
- Use thread pool for recycling threads
- Map and reduce functions are handled like any function passed
  to runAsync, so they either report results through the
  QFutureInterface, or through the return value.
- Automatically deduce the reduce result type

Change-Id: I7a31370c21f8c27b378cd87c3d5974b162449ce1
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>