change source tag in log events from AnyRef to String

- ensure that no “complex” things are attached to a LogEvent (think
  serialization)
- ensure no escaping the “this” reference via LoggingBus during
  constructors (e.g. ActorSystem)
- change it so that
    + Actor/ActorRef are represented by their address
    + Class[_] by simpleName
    + String by itself
- this means that people need to think a little more while deciding how
  “this” should look like in logging (which I think is a good thing)

akka-remote/src/main/scala/akka/remote/RemoteActorRefProvider.scala

@@ -34,7 +34,8 @@ class RemoteActorRefProvider(
   val dispatcher: MessageDispatcher,
   val scheduler: Scheduler) extends ActorRefProvider {
 
-  val log = Logging(eventStream, this)
+  val log = Logging(eventStream, "RemoteActorRefProvider")
+
   val local = new LocalActorRefProvider(settings, rootPath, eventStream, dispatcher, scheduler)
 
   def deathWatch = local.deathWatch