Task dropped in wrong thread when aborting `LocalSet` task

When aborting a task with JoinHandle::abort, the future is dropped in the
thread calling abort if the task is not currently being executed. This is
incorrect for tasks spawned on a LocalSet.

This can easily result in race conditions as many projects use Rc or RefCell
in their Tokio tasks for better performance.

See tokio#3929 for more details.