make waitForDns async

cloudflare is partly broken

src/dig.js

@@ -0,0 +1,41 @@
+'use strict';
+
+exports = module.exports = {
+    resolve,
+};
+
+const assert = require('assert'),
+    constants = require('./constants.js'),
+    dns = require('dns'),
+    safe = require('safetydance'),
+    _ = require('underscore');
+
+// a note on TXT records. It doesn't have quotes ("") at the DNS level. Those quotes
+// are added for DNS server software to enclose spaces. Such quotes may also be returned
+// by the DNS REST API of some providers
+async function resolve(hostname, rrtype, options) {
+    assert.strictEqual(typeof hostname, 'string');
+    assert.strictEqual(typeof rrtype, 'string');
+    assert(options && typeof options === 'object');
+
+    const defaultOptions = { server: '127.0.0.1', timeout: 5000 }; // unbound runs on 127.0.0.1
+    const resolver = new dns.Resolver();
+    options = _.extend({ }, defaultOptions, options);
+
+    // Only use unbound on a Cloudron
+    if (constants.CLOUDRON) resolver.setServers([ options.server ]);
+
+    // should callback with ECANCELLED but looks like we might hit https://github.com/nodejs/node/issues/14814
+    const timerId = setTimeout(resolver.cancel.bind(resolver), options.timeout || 5000);
+
+    const [error, result] = safe(resolver.resolve(hostname, rrtype));
+    clearTimeout(timerId);
+
+    if (error && error.code === 'ECANCELLED') error.code = 'TIMEOUT';
+    if (error) throw error;
+
+    // result is an empty array if there was no error but there is no record. when you query a random
+    // domain, it errors with ENOTFOUND. But if you query an existing domain (A record) but with different
+    // type (CNAME) it is not an error and empty array. for TXT records, result is 2d array of strings
+    return result;
+}