Allow worker tasks to be cancelled by classname

Summary:
Ref T3554. Makes bin/worker cancel --class <classname> work (cancel all tasks with that type).

This is useful in development if your queue is full of a bunch of gunk, and a need has occasionally arisen in production environments (usually "one option is cancel everything and move on").

Test Plan: Ran bin/worker cancel to cancel blocks of tasks by class name.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T3554

Differential Revision: https://secure.phabricator.com/D16267