Asynchronous fetching for queries

[lvca]

Discussed in #177

^{Originally posted by lvca October 29, 2021}
ArcadeDB provides the powerful db.async().command() API to execute a command asynchronously. Unfortunately, the callback has only onOk(Resultset) and onError() but it would be nice having callbacks per single result of the result set. This makes easier integration with reactive frameworks like RxJava.

The proposal is to enrich AsyncResultsetCallback by adding additional method to support both one time call onStart(resultset) and per result with onNext(Result) and onComplete() when done.

public interface AsyncResultsetCallback {
  /**
   * Invoked as soon as the command has been executed.
   *
   * @param resultset Resultset to fetch
   */
  void onStart(ResultSet resultset);

  /**
   * Invoked per single result in the result set.
   *
   * @return true to continue fetching otherwise false. If false is returned, the fetching stops and the {@link #onComplete()} method is never invoked.
   */
  boolean onNext(Result result);

  /**
   * Invoked when the fetching of the entire result set has been completed.
   */
  void onComplete();

  /**
   * Invoked in case of an error in the execution or fetching of the result set.
   *
   * @param exception The exception caught
   */
  void onError(Exception exception);
}