cache auth client

The authClient from the google-auth-library ought to be cached. Otherwise we end up creating a new AuthClient on each request which ends up fetching fresh credentials on every API request. This can potentially double the latency of each API request.

lib/common/util.js

@@ -476,6 +476,10 @@ util.shouldRetryRequest = shouldRetryRequest;
  * @param {function} callback - The callback function.
  */
 function getAuthClient(config, callback) {
+  if (config.authClient) { // cached authClient
+    callback(null, config.authClient);
+    return;
+  }
   var googleAuth = new GoogleAuth();
 
   if (config.keyFile) {
@@ -500,6 +504,7 @@ function getAuthClient(config, callback) {
       authClient = authClient.createScoped(config.scopes);
     }
 
+    config.authClient = authClient;
     callback(null, authClient);
   }
 }