Distributed / multi-document transaction support

[hatspencer]

Motivation:

MongoDB supports distributed transactions since version 4.2, and this has been also available on the driver level for a while. The goal of this PR is to add API support for such distributed / multi document transactions.

Related issues: #242, #158

The newly defined MongoSession is a wrapper implementation around MongoClient but with the added control for start/commit/abort.

Usage examples:

MongoClient#executeTransaction() - compose and execute the provided future in transaction, commit on success, abort on failure automatically using handlers.

public void executeTransactionExample(MongoClient mongoClient) {
  // Match any documents with title=The Hobbit
  JsonObject query = new JsonObject()
    .put("title", "The Hobbit");
  // Set the author field
  JsonObject update = new JsonObject().put("$set", new JsonObject()
    .put("author", "J. R. R. Tolkien"));
  UpdateOptions options = new UpdateOptions().setMulti(true);

  mongoClient.executeTransaction(client -> Future.join(
      client.updateCollectionWithOptions("books", query, update, options),
      client.insert("authors", update)
    ))
    .onFailure(throwable -> System.err.println(throwable.getMessage()))
    .onComplete(res -> {
      if (res.succeeded()) {
        System.out.println("Book and Author updated !");
      } else {
        res.cause().printStackTrace();
      }
    });
}

MongoClient#startSession() - allows for manual control of commit / abort.

public void startSessionExample(MongoClient mongoClient) {
  mongoClient.startSession()
    .flatMap(session -> {
      // Match any documents with title=The Hobbit
      JsonObject query = new JsonObject()
        .put("title", "The Hobbit");
      // Set the author field
      JsonObject update = new JsonObject().put("$set", new JsonObject()
        .put("author", "J. R. R. Tolkien"));
      UpdateOptions options = new UpdateOptions().setMulti(true);

      return session.executeTransaction(client -> Future.join(
        client.updateCollectionWithOptions("books", query, update, options),
        client.insert("authors", update))
      );
    })
    .onFailure(throwable -> System.err.println(throwable.getMessage()))
    .onComplete(res -> {
      if (res.succeeded()) {
        System.out.println("Book and Author updated !");
      } else {
        res.cause().printStackTrace();
      }
    });
}

By default, the underlying session is started with a transaction and closed automatically after commit or abort. The client session's behaviour can be changed by passing a ClientSessionOptions instance. For example:

mongoClient.startSession(new ClientSessionOptions().setCloseSession(false))