Fix the way udp_send_direct() is used

Fix the way udp_send_direct() is used

There were two problems how udp_send_direct() was used:

1. The udp_send_direct() can return ISC_R_CANCELED (or translated error
   from uv_udp_send()), but the isc__nm_async_udpsend() wasn't checking
   the error code and not releasing the uvreq in case of an error.

2. In isc__nm_udp_send(), when the UDP send is already in the right
   netthread, it uses udp_send_direct() to send the UDP packet right
   away.  When that happened the uvreq was not freed, and the error code
   was returned to the caller.  We need to return ISC_R_SUCCESS and
   rather use the callback to report an error in such case.