Eliminate race condition on item removal
Eliminate race condition on item removal

Previously, if enableDeleteTransitions was true and the user removed
items too quickly, one of the componentWillLeave callbacks would be
overwritten and not be called. Now, we check to see if we are
overwriting the callback and call the most recently deleted one
immediately.

dylanmackenzie/react-mutable-list,dylanmackenzie/react-mutable-list