Speed up warmServer by loading pages (and files) asynchronously (#16752)

* Async `new Page`

* Update pages.js

* Update pages.js

* Update pages.js

* Update pages.js

* Update pages.js

* Using mapLimit

* Update pages.js

* Test updates

* Update pages.js

* Move exists to page class

* Test fixes

* Slightly faster localized paths process

tests/routing/redirects.js

@@ -17,8 +17,8 @@ describe('redirects', () => {
     done()
   })
 
-  test('page.redirects is an array', () => {
-    const page = new Page({
+  test('page.redirects is an array', async () => {
+    const page = await Page.init({
       relativePath: 'github/collaborating-with-issues-and-pull-requests/about-branches.md',
       basePath: path.join(__dirname, '../../content'),
       languageCode: 'en'
@@ -26,8 +26,8 @@ describe('redirects', () => {
     expect(isPlainObject(page.redirects)).toBe(true)
   })
 
-  test('dotcom homepage page.redirects', () => {
-    const page = new Page({
+  test('dotcom homepage page.redirects', async () => {
+    const page = await Page.init({
       relativePath: 'github/index.md',
       basePath: path.join(__dirname, '../../content'),
       languageCode: 'en'
@@ -41,7 +41,7 @@ describe('redirects', () => {
   })
 
   test('converts single `redirect_from` strings values into arrays', async () => {
-    const page = new Page({
+    const page = await Page.init({
       relativePath: 'github/collaborating-with-issues-and-pull-requests/about-conversations-on-github.md',
       basePath: path.join(__dirname, '../../content'),
       languageCode: 'en'