stream: Pipe data in chunks matching read data

This creates better flow for large values of lowWaterMark.

lib/_stream_readable.js

@@ -137,8 +137,13 @@ function howMuchToRead(n, state) {
   if (state.objectMode)
     return n === 0 ? 0 : 1;
 
-  if (isNaN(n) || n === null)
-    return state.length;
+  if (isNaN(n) || n === null) {
+    // only flow one buffer at a time
+    if (state.flowing && state.buffer.length)
+      return state.buffer[0].length;
+    else
+      return state.length;
+  }
 
   if (n <= 0)
     return 0;