Add support for cancelling jobs
Add support for cancelling jobs

This commit extends the `Controller` with an API that exposes the
ability of cancelling a job, either enqueued or already running.

akosthekiss/fuzzinator,akosthekiss/fuzzinator,renatahodovan/fuzzinator,renatahodovan/fuzzinator,akosthekiss/fuzzinator,renatahodovan/fuzzinator,renatahodovan/fuzzinator,akosthekiss/fuzzinator