Implement API endpoint for deleting tasks
Implement API endpoint for deleting tasks

This will always stop and delete the entire assignment, because our
architecture does not allow for stopping just one task from a multi-task
assignment.

pyfarm/pyfarm-agent,pyfarm/pyfarm-agent,guidow/pyfarm-agent,pyfarm/pyfarm-agent,guidow/pyfarm-agent,guidow/pyfarm-agent