stream: Short-circuit buffer pushes when flowing

When a stream is flowing, and not in the middle of a sync read, and
the read buffer currently has a length of 0, we can just emit a 'data'
event rather than push it onto the array, emit 'readable', and then
automatically call read().

As it happens, this is quite a frequent occurrence!  Making this change
brings the HTTP benchmarks back into a good place after the removal of
the .ondata/.onend socket kludge methods.

lib/_stream_readable.js

@@ -147,17 +147,24 @@ function readableAddChunk(stream, state, chunk, encoding, addToFront) {
       if (state.decoder && !addToFront && !encoding)
         chunk = state.decoder.write(chunk);
 
-      // update the buffer info.
-      state.length += state.objectMode ? 1 : chunk.length;
-      if (addToFront) {
-        state.buffer.unshift(chunk);
-      } else {
+      if (!addToFront)
         state.reading = false;
-        state.buffer.push(chunk);
-      }
 
-      if (state.needReadable)
-        emitReadable(stream);
+      // if we want the data now, just emit it.
+      if (state.flowing && state.length === 0 && !state.sync) {
+        stream.emit('data', chunk);
+        stream.read(0);
+      } else {
+        // update the buffer info.
+        state.length += state.objectMode ? 1 : chunk.length;
+        if (addToFront)
+          state.buffer.unshift(chunk);
+        else
+          state.buffer.push(chunk);
+
+        if (state.needReadable)
+          emitReadable(stream);
+      }
 
       maybeReadMore(stream, state);
     }