dgram: add support for UDP connected sockets

Added the `dgram.connect()` and `dgram.disconnect()` methods that
associate/disassociate a udp socket to/from a remote address.
It optimizes for cases where lots of packets are sent to the same
address.
Also added the `dgram.remoteAddress()` method to retrieve the associated
remote address.

PR-URL: https://github.com/nodejs/node/pull/26871
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>

test/parallel/test-dgram-connect-send-empty-array.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const common = require('../common');
+
+const assert = require('assert');
+const dgram = require('dgram');
+
+const client = dgram.createSocket('udp4');
+
+client.on('message', common.mustCall((buf, info) => {
+  const expected = Buffer.alloc(0);
+  assert.ok(buf.equals(expected), `Expected empty message but got ${buf}`);
+  client.close();
+}));
+
+client.on('listening', common.mustCall(() => {
+  client.connect(client.address().port,
+                 common.localhostIPv4,
+                 common.mustCall(() => client.send([])));
+}));
+
+client.bind(0);