Use splice() instead of repeated shift() to remove from queue (#106)

Calling splice() is more efficient than shift()ing from the start of the
array up to batchSize(default of 300) times.
The amount of time taken by shift() is proportional to the length of the
array. This would only matter for large arrays.

lib/client/ingest/signal_fx_client.js

@@ -229,10 +229,7 @@ SignalFxClient.prototype._encodeEvent = function (_event) {
 SignalFxClient.prototype.startAsyncSend = function () {
   var _this = this;
   // Send post request in separate thread
-  var datapointsList = [];
-  while (_this.queue.length !== 0 && datapointsList.length < _this.batchSize) {
-    datapointsList.push(_this.queue.shift());
-  }
+  var datapointsList = _this.queue.splice(0, _this.batchSize);
 
   if (datapointsList.length > 0) {
     var dataToSend = _this._batchData(datapointsList);