This message is understood by the connection actors to send data to their
designated destination. The connection actor will respond with
akka.io.Udp.CommandFailed if the send could not be enqueued to the O/S kernel
because the send buffer was full. If the given ack is not of type akka.io.Udp.NoAck
the connection actor will reply with the given object as soon as the datagram
has been successfully enqueued to the O/S kernel.