424043 - IteratingCallback Idle race.

Renamed Next enum to Action, and renamed some constant of the State
enum to avoid confusion with the Action enum.

Simplified succeeded() and failed(Throwable) code,
covering also cases not covered before.

Fixed case SCHEDULED in succeeded() that was returning in case the
compareAndSet failed.

Fixed race in perform(), where 2 threads may execute concurrently
after having returned from process().

Vastly improved javadocs and comments.