Run the forward_cancel on the appropriate zone->loop

Run the forward_cancel on the appropriate zone->loop

If the zone forwards are canceled from dns_zonemgr_shutdown(), the
forward_cancel() would get called from the main loop, which is wrong.
It needs to be called from the matching zone->loop.

Run the dns_request_cancel() via isc_async_run() on the loop associated
with the zone instead of calling the dns_request_cancel() directly from
the main loop.