Driver API was changed with the new requirement in 75c4708, which was released in 1.0.6: the new method tasks_by_state() must be defined in a queue driver. We unable to relax this requirement without potentially loosing of task processing correctness (see #66).

All built-in drivers were updated, but there are external ones and we should give developers clear notification how to update a driver to support queue-1.0.6. Let's give clear error message in case of lack of the method and update the release notes for 1.0.6 with upgrade path and simple example.