- it is customary to use class name for categorizing logs, hence we
should support it; class is taken from logSource.getClass
- update SLF4J module to use logClass as category and set logSource in
MDC "akkaSource"
- add docs
- introduce EmptyLocalActorRef, which is returned for unsuccessful
look-ups of local scope
- this fixes the problem that actors—after their death—can still be
looked up without losing their identity; otherwise behave like
DeadLetterActorRef
- adapt tests accordingly
- make DeathWatchSpec reusable and build remote test from it
- remove several unused imports of LocalActorRef
- use LocalRef/RemoteRef in pattern matches where applicable: these are
marker traits for a ref’s scope; InternalActorRef mandates a scope as
per its self-type
