Simplify locking in URLHandlers

The URLHandlers class has a lot of complicated synchronization which makes it hard to predict its behaviour wrt to deadlocks. Currently this class synchronizes on 5 objects: the URL.class object, the this object, m_classloaderToFrameworkLists, m_frameworks and m_builtIn.

While it might be that all these locks are needed, it would be nice to investigate whether this class could be refactored to use less synchronization so that there are less chances of deadlocks.