SPARK-4305 [BUILD] yarn-alpha profile won't build due to network/yarn module

SPARK-3797 introduced the `network/yarn` module, but its YARN code depends on YARN APIs not present in older versions covered by the `yarn-alpha` profile. As a result builds like `mvn -Pyarn-alpha -Phadoop-0.23 -Dhadoop.version=0.23.7 -DskipTests clean package` fail.

The solution is just to not build `network/yarn` with profile `yarn-alpha`.

Author: Sean Owen <sowen@cloudera.com>

Closes #3167 from srowen/SPARK-4305 and squashes the following commits:

88938cb [Sean Owen] Don't build network/yarn in yarn-alpha profile as it won't compile
[SPARK-4282][YARN] Stopping flag in YarnClientSchedulerBackend should be volatile

In YarnClientSchedulerBackend, a variable "stopping" is used as a flag and it's accessed by some threads so it should be volatile.

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

Closes #3143 from sarutak/stopping-flag-volatile and squashes the following commits:

58fdcc9 [Kousuke Saruta] Marked stoppig flag as volatile