clean up view and related object shutdown processes

The shutdown processes in dns_view, dns_resolver, dns_requestmgr and dns_adb were quite complex, involving circular object references (i.e., the view is attached to the ADB which is attached to the view) and whenshutdown callback events. This can be simplified by correctly using dns_view_weakattach() and dns_view_weakdetach() and eliminating mutual dependencies between these objects during the shutdown process.