fix: improved performance of Pipeline.exec (#991)

NOTE:  @flunderpero improved performance of Pipeline.exec, reset buffers and made code more readable

Motivation for the change:

```
Execution cost of Pipeline.exec can be reduced to 50% or lower. This is
accomplished by first filling up an Array of Buffers and then calling Buffer.concat
instead of calling Buffer.concat for each command.
In our use-case (~3000 commands in a single Pipeline) wall-clock-runtime went
down from 1.3sec to 500ms.
```

lib/pipeline.ts

@@ -302,7 +302,8 @@ Pipeline.prototype.exec = function(callback: CallbackFunction) {
     .then(execPipeline);
 
   function execPipeline() {
-    let data: Buffer | string = "";
+    let data = "";
+    let buffers: Buffer[];
     let writePending: number = (_this.replyPending = _this._queue.length);
 
     let node;
@@ -319,25 +320,38 @@ Pipeline.prototype.exec = function(callback: CallbackFunction) {
           bufferMode = true;
         }
         if (bufferMode) {
-          data = Buffer.concat([
-            typeof data === "string" ? Buffer.from(data, "utf8") : data,
+          if (!buffers) {
+            buffers = [];
+          }
+          if (typeof data === "string") {
+            buffers.push(Buffer.from(data, "utf8"));
+            data = undefined;
+          }
+          buffers.push(
             typeof writable === "string"
               ? Buffer.from(writable, "utf8")
               : writable
-          ]);
+          );
         } else {
           data += writable;
         }
         if (!--writePending) {
+          let sendData: Buffer | string;
+          if (buffers) {
+            sendData = Buffer.concat(buffers);
+          } else {
+            sendData = data;
+          }
           if (_this.isCluster) {
-            node.redis.stream.write(data);
+            node.redis.stream.write(sendData);
           } else {
-            _this.redis.stream.write(data);
+            _this.redis.stream.write(sendData);
           }
 
           // Reset writePending for resending
           writePending = _this._queue.length;
           data = "";
+          buffers = undefined;
           bufferMode = false;
         }
       }