Improve ATSv15 performance for DAG entities read calls.

TimelineCachePluginImpl generates TimelineEntityGroupId.newInstance(appId, appId.toString()) as an EntityGroupId for dag entities. This results entities in file based on application EntityGroupId to loaded by ATS during a read operation. This file can become very large in long running AM and causes timeout and delays in ATS.

Removing this id, reduces this parsing time and prevents the application entities from being loaded into cache.