Fix bango logout (bug 1105076,1105076,1049004)

* Use logged_in_user as the key for provider logouts. 
* Set the localStorage to a logout+timestamp when we logout (This causes the provider logout later at the point we know the provider).

r? @kumar303 @washort 
I think it would be simpler if you did this:

    if (existingUser && existingUser !== currentUser) {
        // log a message and call trackEvent
        needsLogout = true;
    }
    if (needsLogout) {
         ...
    }