Fix issue #597

This commit changes how scheduler breadth first works. The new version uses
a NOT EXISTS function to take into account the participation of the user, as
well as distinguish between authenticated and anonymous users.
Remove old code for scheduler. Refs #597
Increase coverage

This commit removes from coverage the case for local testing, where the IP
address is set up by the backend.
Increase coverage testing a new case
Update the scheduler documentation
Update changelog
Merge pull request #649 from PyBossa/issue-597-scheduler

Issue 597 scheduler