Add Closeable to Cluster

This is a simple quality of life improvement if a user only needs to execute a handful of operations we can perform the disconnect operation on behalf of them. This enables try-with-resource like this:

try (Cluster cluster = Cluster.connect("127.0.0.1", "user", "password")) {

  Bucket bucket = cluster.bucket("travel-sample");

  bucket.waitUntilReady(Duration.ofSeconds(10));

}