Adding util method to Java API. Related #3671.

* Converts java.util.Map to scala.collection.immutable.Map
MDC mechanism is now available for simple message methods as well as for templates. Fixes #3671.

* The message wrapper case class was moved to akka-actor.
* Now the wrapping happens inside the LoggingAdapter not in the caller.
* MDC values populator was redefined as a function which the application can specify when it creates the LoggingAdapter.
* A test was introduced to verify that the new API is Java friendly.
* BusLogging now supports this new feature and ActorLogging was adapted to hook the MDC function when it creates the LoggingAdapter