fix issue: event listener is being registered multiple times

[zirkelc]

I experienced an issue in a multi databases setup:

When Neo4j starts it creates only one instance of MyExtensionFactory. Then it starts each database, and each database calls the MyExtensionFactory.newInstance method. However, the database is stored on the object property MyExtensionFactory.db and access within the subclass MyAdapter.start method, which means it accesses a property from a parent object that changes this property on every MyExtensionFactory.newInstance call.