Fix invalid RMApp transition from NEW to FINAL_SAVING

YARN-891 augments the RMStateStore to store information on completed applications. In the process, it adds transitions from NEW to FINAL_SAVING. This leads to the RM trying to update entries in the state-store that do not exist. On ZKRMStateStore, this leads to the RM crashing.

Previous description:
ZKRMStateStore fails to handle updates to znodes that don't exist. For instance, this can happen when an app transitions from NEW to FINAL_SAVING. In these cases, the store should create the missing znode and handle the update.