Refactor startup/shutdown for tests

* `src/node/server.js` can now be run as a script (for normal
    operation) or imported as a module (for tests).
  * Move shutdown actions to `src/node/server.js` to be close to the
    startup actions.
  * Put startup and shutdown in functions so that tests can call them.
  * Use `await` instead of callbacks.
  * Block until the HTTP server is listening to avoid races during
    test startup.
  * Add a new `shutdown` hook.
  * Use the `shutdown` hook to:
      * close the HTTP server
      * call `end()` on the stats collection to cancel its timers
      * call `terminate()` on the Threads.Pool to stop the workers
  * Exit with exit code 0 (instead of 1) on SIGTERM.
  * Export the HTTP server so that tests can get the HTTP server's
    port via `server.address().port` when `settings.port` is 0.

src/node/utils/Minify.js

@@ -418,3 +418,7 @@ exports.minify = minify;
 
 exports.requestURI = requestURI;
 exports.requestURIs = requestURIs;
+
+exports.shutdown = async (hookName, context) => {
+  await threadsPool.terminate();
+};