Fix for issue 3011: 

Created custom DirectoryThreadGroup class that all DirectoryThreads will be a member of. All unhandled exceptions from DirectoryThreads will be handled by the handler in DirectoryThreadGroup. This keeps OpenDS from having to install a JVM wide uncaughtException handler.

Changed the configuration object reference a member variable in EntryCacheMonitorProvider instead of a static. This lets GC collected all ConfigEntry and other related objects after OpenDS is shutdown.