Avoid race condition in scartchpad eraser tool

Summary:
Also fade out stroke because that looks better.

Fixes https://app.getsentry.com/khanacademyorg/prod-js/group/15018626/.

Test Plan: Changed fade-out time to 1000 ms and erased two things concurrently; things didn't blow up.

Reviewers: eater, alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D6838