Variable DataflowNode.fired refactored into pin bit;
other minor cahges.
1. core/DataflowNode split in DFNode (no use of thread pool) and DFActor
2. core/Link and its derivatives removed, to get rid of low-level optimizations and keep the library conmceptually clean.
CallbackFuture and CallbackPromise combined in a single class ListenableFuture.
  node class hierarchy refactored
nio* projects adjusted to the core project changes
unused constants deleted
Immediate executor made safe for long chains