Properly unload jar extension on uninstallation without restarting

At the end of an uninstallation job, the ClassLoader associated to the modified namespaces are dropped and new ClassLoader are reloaded with the remaining extensions. The related components are reregistered.

For this the works properly, any extensions that takes references to extensions from a sibling namespace or core components that takes references to extensions, should take care to release their reference to these extensions when they are unregistered (and maybe take a new reference when a new registration occurs)

Extension should be prepare to be disposed and reloaded at anytime.