change from isc_nmhandle_ref/unref to isc_nmhandle attach/detach

change from isc_nmhandle_ref/unref to isc_nmhandle attach/detach

Attaching and detaching handle pointers will make it easier to
determine where and why reference counting errors have occurred.

A handle needs to be referenced more than once when multiple
asynchronous operations are in flight, so callers must now maintain
multiple handle pointers for each pending operation. For example,
ns_client objects now contain:

        - reqhandle:    held while waiting for a request callback (query,
                        notify, update)
        - sendhandle:   held while waiting for a send callback
        - fetchhandle:  held while waiting for a recursive fetch to
                        complete
        - updatehandle: held while waiting for an update-forwarding
                        task to complete

(cherry picked from commit 57b4dde9749c88d21d1dc8afd22201224cf83cab)