remove uTP delayed ack and instead send acks when the udp socket has been drained. simplify the udp socket to use null_buffers and allocate less memory for buffers. this also eliminated the race condition when resizing the udp socket receive buffer which greatly simplified it

src/session_impl.cpp

@@ -644,6 +644,7 @@ namespace aux {
 		, m_udp_socket(m_io_service
 			, boost::bind(&session_impl::on_receive_udp, this, _1, _2, _3, _4)
 			, boost::bind(&session_impl::on_receive_udp_hostname, this, _1, _2, _3, _4)
+			, boost::bind(&session_impl::on_udp_socket_drained, this)
 			, m_half_open)
 		, m_utp_socket_manager(m_settings, m_udp_socket
 			, boost::bind(&session_impl::incoming_connection, this, _1))
@@ -2500,6 +2501,14 @@ namespace aux {
 		}
 	}
 
+	// this is called every time all packets have been read from
+	// the udp socket. The utp_socket_manager uses this event to
+	// trigger a flush of deferred ACKs
+	void session_impl::on_udp_socket_drained()
+	{
+		m_utp_socket_manager.socket_drained();
+	}
+
 	void session_impl::async_accept(boost::shared_ptr<socket_acceptor> const& listener, bool ssl)
 	{
 		TORRENT_ASSERT(!m_abort);