Verticle api improvements

[vietj]

A new base class implementing io.vertx.core.Verticle called VerticleBase that simplifies the creation of a verticle and uses method returning futures. Methods that should not be overridden by users are final.

// Current way
vertx.deployVerticle(new AbstractVerticle() {
  private HttpServer server;
  public void start(Promise<Void> promise) {
    server = vertx.createHttpServer().requestHandler(requestHandler);
    server
      .listen()
      .<Void>mapEmpty().onComplete(promise);
  }
});

// New way
vertx.deployVerticle(new VerticleBase() {
  private HttpServer server;
  public Future<Void> start() {
    server = vertx.createHttpServer().requestHandler(requestHandler);
    return server.listen();
  }
});

Open questions:

• how do we handle sync start (virtual thread actually use it) ?