Fixing the Logout/currentUser race condition.

[MonirAbuHilal]

This code change fixes a race condition in the overriden Parse.User.logOut() method.

The logOut() method is supposed to reset the current user to null but the code path is not waiting for the oldLogOut() promise result to update the currentUser state in the local subscriptions module (LocalSubscriptions.js) which causes the LocalSubscription.currentUser.update() method to call the setCurrentUser(...) with the stale user object.

Example

Parse.User.logOut()
          .then(() => Parse.User.currentAsync())
          .then((user) => { console.log("Current User", user); });

The JS code snippet above is supposed to log "null" as the value of the user parameter, but it's logging the old stale user object all the time.

[MonirAbuHilal]

Fixes: #161

[TonyTangAndroid]

When this pull request will be merged?