YarnClient will automatically grab a timeline DT for the application and pass it to the app AM. Now the timeline DT renew is still dummy. If an app is running for more than 24h (default DT expiry time), the app AM is no longer able to use the expired DT to communicate with the timeline server. Since RM will cache the credentials of each app, and renew the DTs for the running app. We should provider renew hooks similar to what HDFS DT has for RM, and set RM user as the renewer when grabbing the timeline DT.