[MRG] Automatically group short tasks in batches

This is based on an online evaluation of the batch completion time to dynamically tune the size of the batch of tasks to dispatch at once to a worker.

TODO:

- [x] ensure tests pass on windows
- [x] update the change log
- [X] test with scikit-learn
I think it was from debugging and I forgot about it. It should definitely not be skipped and I think we did check that it passes at some point.