Major Description
Currently Livy queries Yarn applications by applicationType : Spark. This will put heavy load on Yarn clusters if there are thousands or more Spark application in all states (running, finished, failed, queued etc.).
A better approach would be to query the applications by tags in addition to job type since Livy only needs to track application with certain application tags. However, YarnClient does not expose any API to query applications by tags.
We can extend the YarnClientImpl which would allow us to use protected rmClient in YarnClient and we can implement getApplications method with GetApplicationsRequest where we can include tags.
