update: use array in multi tracer implementation

(to be squashed)
update: improve tracer documentation

(to be squashed)
update: add system.hasTracer checks before all tracer calls

(to be squashed)
update: use orNull rather than getOrElse(null)

(to be squashed)
update: use deadLetters for no sender, rather than null, on actorTold

(to be squashed)