feat: federated learning demo on the actor framework

- supervisor: ignore stale death messages (OneForAll restart storm),
  drop normally-exited children instead of respawning, add a
  restart-rate limit
- register actors before their thread starts so UUID dispatch and
  getSelf cannot race the spawn
- killActor now kills the thread so links see Killed
- fulfill connection promises with the pool winner on simultaneous
  connects
- clamp the PaySim test split on small datasets and split shared
  transaction types across trainers in non-IID partitioning

src/Control/Actor.hs

@@ -5,6 +5,8 @@ module Control.Actor
   , module Control.Actor.Core
   , module Control.Actor.Supervision
   , module Control.Actor.Network
+  -- Initialization
+  , initRuntime
   -- Demo
   , pingActor
   , forwardActorWithCell