Prevent the double xfrin_fail() call
When we are reading from the xfrin socket, and the transfer would be
shutdown, the shutdown function would call
xfrin_fail() which in turns
xfrin_cancelio() that causes the read callback to be invoked
ISC_R_CANCELED status code and that caused yet another
The proper fix here is to just set the shuttingdown flag (so, it's properly caught by the callbacks) and call xfrin_cancelio() from dns_xfrin_shutdown() function.
Closes #2630 (closed)