=act #15488 add @volatile on logLevel

This PR fixes the memory visibility issue introduced in #15488.

Summary:
* made `logLevel` @volatile
* Perf benchmark and results are attached.

Please review @viktorklang
How about skipping the instantiation of an ActorSystem and create a BusLogging directly? https://github.com/akka/akka/blob/master/akka-actor/src/main/scala/akka/event/Logging.scala#L511