try to avoid a media_addon_server crash on quit (bug #5863).

* BMediaRosterEx::ReleaseNodeAll(): if media_server isn't available,
try to contact the node locally.
* BMediaRoster::UnregisterNode(): if media_server isn't available,
try to get the addon_id from the local node.
* BMediaNode::AddOn() as a pure virtual methode isn't available in the
BMediaNode destructor. Workaround that by calling UnregisterNode()
in DeleteHook() instead of the destructor.
* MediaAddonServer::_DestroyInstantiatedFlavors(): give time to each
flavor to clean up.