Don't set the offloaded work result from main thread

The xfrin_recv_done() was accessing xfr->result where we stored the result of the offloaded work from a thread that could receive data while processing the transfer on the offloaded thread.

Completely remove the offloaded result from the dns_xfrin_t structure and keep it local for *xfr_apply() and *xfr_apply_done() as the failure is already recorded in .shutdown_result and we now that the processing has failed because .shuttingdown has been already set.

Closes #4380 (closed)