=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
It means that no-one can do check-then-act operations on _logLevel because there is no atomic operations exposed. If all writes are done under the guard, it is safe to do check-then-act as long as you do it within the guard. The hoisting of the read of _logLevel as proposed earlier for example. Or imagine if someone writes _logLevel while iterating over the logLevels in the `setLogLevel` method?