MINOR: connection: check for connection validation earlier

MINOR: connection: check for connection validation earlier

In conn_fd_handler() we used to first give a chance to the send()
callback to try to send data and validate the connection at the same
time. But since 1.9 we do not call this callback anymore inline, it's
scheduled. So let's validate the connection ealier so that all other
decisions can be taken based on this confirmation. This may notably
be useful to the xprt_done_cb() to know that the connection was
properly validated.