WorkflowInstance.start() is not thread-safe #1802

WorkflowMutableInstance#startExecution guards single-execution with a non-atomic check-then-act:

  future = futureRef.get();
  if (future == null) {
      future = runnable.get()...;   // starts task processing, fires WorkflowStartedEvent
      futureRef.set(future);
  }
  return future;

Two threads calling start() at the same time can both see futureRef.get() == null and both run the startup side effects before either set() wins.