stream: remove usage of *State.highWaterMark

Replaced _readableState.highWaterMark with a .readableHighWaterMark
getter and _writableState.highWaterMark with a .writableHighWaterMark
getter.
The getters are non-enumerable because they break some prototype
manipulation that happen in the ecosystem.

Ref: https://github.com/nodejs/node/issues/445.

PR-URL: https://github.com/nodejs/node/pull/12860
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

lib/_http_server.js

@@ -373,13 +373,13 @@ function connectionListener(socket) {
 function updateOutgoingData(socket, state, delta) {
   state.outgoingData += delta;
   if (socket._paused &&
-      state.outgoingData < socket._writableState.highWaterMark) {
+      state.outgoingData < socket.writeHWM) {
     return socketOnDrain(socket, state);
   }
 }
 
 function socketOnDrain(socket, state) {
-  var needPause = state.outgoingData > socket._writableState.highWaterMark;
+  var needPause = state.outgoingData > socket.writeHWM;
 
   // If we previously paused, then start reading again.
   if (socket._paused && !needPause) {
@@ -569,7 +569,7 @@ function parserOnIncoming(server, socket, state, req, keepAlive) {
   // pipelined requests that may never be resolved.
   if (!socket._paused) {
     var ws = socket._writableState;
-    if (ws.needDrain || state.outgoingData >= ws.highWaterMark) {
+    if (ws.needDrain || state.outgoingData >= socket.writableHighWaterMark) {
       socket._paused = true;
       // We also need to pause the parser, but don't do that until after
       // the call to execute, because we may still be processing the last