ENH add get_worker_rank with unique rank

[tomMoral]

Add loky.get_worker_rank function that returns a unique number between 0 and executor.max_workers - 1 in the workers.
This functionality could be used to better assign resources such as GPU in parallel process.

TODO:

• implement the rank
• Add some test that check that it works fine with timeout and resize.
• Add some doc
• Add an entry in what's new.

[tomMoral]

This PR is another implementation of the feature proposed in #261 .
The main difference is that the map linking pid and worker id is send back with each task, to notify the worker in case a resize occurred and his rank should now be smaller than the initial one.