workerpool: Use itertools.count instead of manual counting

Instead of having to explicitely increment the value (“… += 1”), a call
to next() is enough. These numbers should in no case be re-used (they
are used for ordering tasks). Using “itertools.count” is useful here as
it guarantees that a returned number won't be returned another time.
Manual code for this could, over the course of time, gain unintended
bugs.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Bernardo Dal Seno <bdalseno@google.com>