http2: 10% perf improvement.

PR-URL: #20
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/internal/http2.js

@@ -450,9 +450,7 @@ class Http2Stream extends Duplex {
         throw util._errnoException(err, 'write', req.error);
       this._bytesDispatched += req.bytes;
     } else {
-      this.once('handle', () => {
-        this._write(data, encoding, cb);
-      });
+      this.once('handle', onHandleWrite(data, encoding, cb));
     }
   }
 
@@ -483,6 +481,12 @@ class Http2Stream extends Duplex {
   }
 }
 
+function onHandleWrite(data, encoding, cb) {
+  return function onWriteFinished() {
+    this._write(data, encoding, cb);
+  };
+}
+
 class Http2Session extends EventEmitter {
   constructor(type, options, socket) {
     super();
@@ -713,6 +717,7 @@ class Http2Outgoing extends Writable {
     this[kFinished] = false;
     this[kHeadersSent] = false;
     this.on('pipe', onHttp2OutgoingPipe);
+    this.bufferedCallback = null;
   }
 
   get stream() {
@@ -791,11 +796,8 @@ class Http2Outgoing extends Writable {
       chunk = Buffer.from(chunk, encoding);
     if (this.stream) {
       this[kBeginSend]();
-      this.stream.write(chunk, encoding, () => {
-        this[kResume]();
-        if (typeof callback === 'function')
-          callback();
-      });
+      this.bufferedCallback = callback;
+      this.stream.write(chunk, encoding, outWriteResume);
     } else {
       this[kFinished] = true;
       callback();
@@ -826,6 +828,14 @@ class Http2Outgoing extends Writable {
   }
 }
 
+function outWriteResume() {
+  this[kResume]();
+  const callback = this.bufferedCallback;
+  this.bufferedCallback = null;
+  if (typeof callback === 'function')
+    callback();
+}
+
 // Represents an HTTP/2 server response message
 class Http2ServerResponse extends Http2Outgoing {
   constructor(stream, options) {