Use Arc<Mutex<T>> rather than Rc<RefCell<T>> in DiskStorage.
Use Arc<Mutex<T>> rather than Rc<RefCell<T>> in DiskStorage.

This keeps DiskStorage Sync + Send and therefore Authenticator Sync +
Send. The DiskStorage was threadsafe because JSONTokens contains a Mutex
around all the Rc<RefCell<T>> objects, but there's no way to prove to
the type system that none of the Rc's get cloned to an alias used
outside the Mutex so it's not provably safe. I'll probably reevaluate
the design here, but in the meantime the double locking is fine.

dermesser/yup-oauth2,dermesser/yup-oauth2