Implement app clustering (#17752)

* Install throng for easy cluster management
* Extract the Express app construction into its own file
* Switch server.js to use app clustering for deployed environments
* Worker count is based on the lesser of process.env.WEB_CONCURRENCY and the count of CPUs
* Reading clustered output is difficult, let's prefix the std{out,err} streams

Co-authored-by: Jason Etcovitch <jasonetco@github.com>

tests/routing/redirects.js

@@ -1,7 +1,7 @@
 const path = require('path')
 const { isPlainObject } = require('lodash')
 const supertest = require('supertest')
-const app = require('../../server')
+const app = require('../../lib/app')
 const enterpriseServerReleases = require('../../lib/enterprise-server-releases')
 const nonEnterpriseDefaultVersion = require('../../lib/non-enterprise-default-version')
 const Page = require('../../lib/page')