add deploy:cleanup_lingering_releases task

Adding a new task for cleaning up lingering releases. Detailed comments as to its purpose in the code.
It doesn't cover the case where there are differing currents, which is a bit trickier to deal with. That shouldn't happen in theory as long as when you do a canary deploy you roll it back. I'd be happy to try and tackle that more automatically later, but for now this is a step in the right direction.