ResourceManger.clusterTimeStamp should be reset when RM transitions to active

ResourceManager.clusterTimeStamp is used to generate application-ids.

Currently, when the RM transitions to active-standby-active back and forth, the clusterTimeStamp stays the same leading to apps getting the same ids as jobs from before. This leads to other races in staging directory etc.

To avoid this, it is better to set it on every transition to Active.