=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.

Related:
* Also benched replacing the guard with an read-write guard, but seemed to have the opposite effect as expected actually... ( https://github.com/akka/akka/commit/5eba9fceef884b16c8764a78d0d8bb274dfbc830#diff-47ea75a55aa7f26d9b488bfb764f9d79R32 ), did not dig deeper into it for now.

Please review @viktorklang
That's your comment originally :-)
I thought that because in `setLogLevel` we actually never write to loggers, so could be a readLock, but I have not seen much gains from doing so so far (perhaps I should have benched with more writer threads).