fix(server): address domain performance regression with Node v12.x

lib/server.js

@@ -2,7 +2,6 @@
 
 'use strict';
 
-var domain = require('domain');
 var EventEmitter = require('events').EventEmitter;
 var http = require('http');
 var https = require('https');
@@ -27,6 +26,7 @@ var customErrorTypes = require('./errorTypes');
 var patchRequest = require('./request');
 var patchResponse = require('./response');
 
+var domain;
 var http2;
 
 patchResponse(http.ServerResponse);
@@ -930,6 +930,14 @@ Server.prototype._onRequest = function _onRequest(req, res) {
     // It has significant negative performance impact
     // Warning: this feature depends on the deprecated domains module
     if (self.handleUncaughtExceptions) {
+        // In Node v12.x requiring the domain module has a negative performance
+        // impact. As using domains in restify is optional and only required
+        // with the `handleUncaughtExceptions` options, we apply a singleton
+        // pattern to avoid any performance regression in the default scenario.
+        if (!domain) {
+            domain = require('domain');
+        }
+
         var handlerDomain = domain.create();
         handlerDomain.add(req);
         handlerDomain.add(res);