LightArrayRevolverScheduler, see #2904

- based on a wheel (AtomicReferenceArray) from which atomic
  single-linked lists dangle
- no locks
- deterministic tests due to overridable time source
- also bring docs up to date

akka-docs/rst/scala/code/docs/actor/SchedulerDocSpec.scala

@@ -29,7 +29,7 @@ class SchedulerDocSpec extends AkkaSpec(Map("akka.loglevel" -> "INFO")) {
     expectMsg(1 second, "foo")
 
     //#schedule-one-off-thunk
-    //Schedules a function to be executed (send the current time) to the testActor after 50ms
+    //Schedules a function to be executed (send a message to the testActor) after 50ms
     system.scheduler.scheduleOnce(50 milliseconds) {
       testActor ! System.currentTimeMillis
     }