Fix the way tcp_send_direct() is used

Fix the way tcp_send_direct() is used

There were two problems how tcp_send_direct() was used:

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

2. In isc__nm_tcp_send(), when the TCP send is already in the right
   netthread, it uses tcp_send_direct() to send the TCP 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.