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.

doc/api/hooks_server-side.md

@@ -10,6 +10,28 @@ Things in context:
 
 Use this hook to receive the global settings in your plugin.
 
+## shutdown
+Called from: src/node/server.js
+
+Things in context: None
+
+This hook runs before shutdown. Use it to stop timers, close sockets and files,
+flush buffers, etc. The database is not available while this hook is running.
+The shutdown function must not block for long because there is a short timeout
+before the process is forcibly terminated.
+
+The shutdown function must return a Promise, which must resolve to `undefined`.
+Returning `callback(value)` will return a Promise that is resolved to `value`.
+
+Example:
+
+```
+// using an async function
+exports.shutdown = async (hookName, context) => {
+  await flushBuffers();
+};
+```
+
 ## pluginUninstall
 Called from: src/static/js/pluginfw/installer.js