PostgresClient futurisation for Vert.x 4, Part 1

This is part 1. Part 2 is RMB-814.

Example:

public interface NetClient {

  // Since 3.0
  void connect(int port, String host, Handler<AsyncResult<NetSocket>> handler);

  // New in 4.0
  Future<NetSocket> connect(int port, String host);
}

This hybrid model has already been implemented in

• Vert.x Core library: https://vertx.io/blog/eclipse-vert-x-4-milestone-1-released/#core-futurisation
• vertx-auth, vertx-web, vertx-mqtt, vertx-cassandra-client, vertx-redis-client, vertx-kakfa-client, vertx-amqp-client https://vertx.io/blog/eclipse-vert-x-4-milestone-2-released/#futurisation
• vertx-service-discovery, vertx-config, vertx-circuit-breaker https://vertx.io/blog/eclipse-vert-x-4-milestone-3-released/#futurisation
• Vert.x Shell, Vert.x Mail Client, Vert.x Consul Client, Vert.x RabbitMQ Client, Vert.x Stomp, Vert.x Mongo Client https://vertx.io/blog/eclipse-vert-x-4-milestone-4-released/#futurisation

The methods that return a future allow the more easy to use and more intuitive way for async coordination, example code from vertx-core doc:

Future<HttpServer> httpServerFuture = Future.future(promise -> httpServer.listen(promise));

Future<NetServer> netServerFuture = Future.future(promise -> netServer.listen(promise));

CompositeFuture.all(httpServerFuture, netServerFuture).onComplete(ar -> {
  if (ar.succeeded()) {
    // All servers started
  } else {
    // At least one server failed
  }
});

Task:
For each PostgresClient method that takes a Handler<AsyncResult<T>> parameter create a function without such parameter that returns a Future<T>.

This can easily been split into several sub-tasks because there are many independent methods. This allows for smaller pull requests and early code review feedback.

For details see
https://github.com/folio-org/raml-module-builder/blob/master/doc/futurisation.md

Implementation notes:

• RMB-741 "PostgresClient.withTransaction" should be complete before we start with this.
• RMB-758 "SQLConnection.x(...) for each PostgresClient.x(AsyncResult<SQLConnection> tx, ...)" is a sub-task of this issue and should be worked on at the same time.
• The existing method should call the new method and do .onComplete(promise) to avoid creating yet another promise, for details see https://github.com/folio-org/raml-module-builder/blob/master/doc/futurisation.md#future-as-return-value