Convert backend code to ES modules

Bundling code now with Rollup for its flat bundling and tree shaking
support. Front end code still goes through Webpack for browser support.

lib/cluster.js

@@ -18,8 +18,9 @@
  */
 "use strict";
 
-const cluster = require("cluster");
-const logger = require("./logger");
+import cluster from "cluster";
+import { cpus } from "os";
+import * as logger from "./logger";
 
 let respawnTimestamp = 0;
 let tooManyCrashesTimestamp = 0;
@@ -62,7 +63,7 @@ function restartWorker(worker, code, signal) {
   }, respawnTimestamp - now);
 }
 
-function start(workerCount, servicePort, serviceAddress) {
+export function start(workerCount, servicePort, serviceAddress) {
   cluster.on("listening", (worker, address) => {
     if (
       (address.addressType === 4 || address.addressType === 6) &&
@@ -80,16 +81,14 @@ function start(workerCount, servicePort, serviceAddress) {
 
   cluster.on("exit", restartWorker);
 
-  if (!workerCount) workerCount = Math.max(2, require("os").cpus().length);
+  if (!workerCount) workerCount = Math.max(2, cpus().length);
 
   for (let i = 0; i < workerCount; ++i) cluster.fork();
 }
 
-function stop() {
+export function stop() {
   cluster.removeListener("exit", restartWorker);
   for (const pid in cluster.workers) cluster.workers[pid].kill();
 }
 
-exports.start = start;
-exports.stop = stop;
-exports.worker = cluster.worker;
+export const worker = cluster.worker;