Locally cache oauth tokens.

This change is for both App Engine and Managed VMs so that these apps can scale without running into the app_identity_service quota limit due to calling appengine.AccessToken too frequently.  An added benefit of caching is that calls to Google APIs will be significantly sped up due to removing the round-trip calls to the api_identity_service.

google/example_test.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build appenginevm !appengine
+
 package google_test
 
 import (