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/db/DB.js

@@ -71,3 +71,8 @@ exports.init = function() {
     });
   });
 }
+
+exports.shutdown = async (hookName, context) => {
+  await exports.doShutdown();
+  console.log('Database closed');
+};