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/utp_socket_manager.cpp

@@ -275,6 +275,27 @@ namespace libtorrent
 		return false;
 	}
 
+	void utp_socket_manager::socket_drained()
+	{
+		// flush all deferred acks
+		
+		std::vector<utp_socket_impl*> deferred_acks;
+		m_deferred_acks.swap(deferred_acks);
+		for (std::vector<utp_socket_impl*>::iterator i = deferred_acks.begin()
+			, end(deferred_acks.end()); i != end; ++i)
+		{
+			utp_socket_impl* s = *i;
+			utp_send_ack(s);
+		}
+	}
+
+	void utp_socket_manager::defer_ack(utp_socket_impl* s)
+	{
+		TORRENT_ASSERT(std::find(m_deferred_acks.begin(), m_deferred_acks.end(), s)
+			== m_deferred_acks.end());
+		m_deferred_acks.push_back(s);
+	}
+
 	void utp_socket_manager::remove_socket(boost::uint16_t id)
 	{
 		socket_map_t::iterator i = m_utp_sockets.find(id);