[SPARK-3567] appId field in SparkDeploySchedulerBackend should be volatile

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2428 from sarutak/appid-volatile-modification and squashes the following commits:

c7d890d [Kousuke Saruta] Added volatile modifier to appId field in SparkDeploySchedulerBackend