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/RemoteConnectionManager.scala

@@ -25,7 +25,7 @@ class RemoteConnectionManager(
   initialConnections: Map[RemoteAddress, ActorRef] = Map.empty[RemoteAddress, ActorRef])
   extends ConnectionManager {
 
-  val log = Logging(system, this)
+  val log = Logging(system, "RemoteConnectionManager")
 
   // FIXME is this VersionedIterable really needed? It is not used I think. Complicates API. See 'def connections' etc.
   case class State(version: Long, connections: Map[RemoteAddress, ActorRef])