Make account.sync_lock() gevent-safe.

Summary: Thanks @emfree for pointing out this bug.

Test Plan:
./runtests, manually fired up a sync and stopped it, seems OK. I
can't reproduce the error you were getting stopping a sync on master though,
does this fix-in-theory work for you?

Reviewers: emfree

Reviewed By: emfree

CC: emfree

Differential Revision: https://review.inboxapp.com/D109