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
Merge pull request #1027 from akka/wip-2904-timer-∂π

first cut of new AkkaTimer, see #2904