Convert managers to singletons.
Fix #6
New pattern overview:
All managers & ManagerLocator are singletons.
New instance of managers (but not locator) created on server start.
All event listeners reference static methods that, within them, get the singleton instance.
Event listeners are registered once, in each Manager's init() function.
previous ManagerLocator.init function converted to ManagerLocator.onServerStart
Technically WorldDataManager isn't a singleton, but it effectively is, via ManagerLocator.
