code: public RiakNode setExecutor(ScheduledExecutorService executor)
      {
          stateCheck(State.CREATED);
          if (this.executor != null)
          {
              throw new IllegalArgumentException("Executor already set");
          }
          this.executor = executor;
          return this;
      }
docs: Sets the {@link ScheduledExecutorService} for this Node and its pool(s).
  
  @param executor - the ScheduledExecutorService to use.
  @return a reference to this RiakNode
  @throws IllegalArgumentException if it was already set via the builder.
  @throws IllegalStateException    if the node has already been started.
  @see Builder#withExecutor(java.util.concurrent.ScheduledExecutorService)