(core) tweak handler for aborted connections to work on modern node

Summary:
It became hard to detect aborted connections in node 16.
In node 14, req.on('close', ...) did the job. Thid diff adds a
work-around, until a better way is discovered or added.
Aborting a req will typically lead to 'close' being called
on the response, without writableFinished being set.

 - https://github.com/nodejs/node/issues/38924
 - https://github.com/nodejs/node/issues/40775

Test Plan:
existing DocApiForwarder test passes; manually
checking on various node versions.

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Differential Revision: https://phab.getgrist.com/D3923

app/gen-server/lib/DocApiForwarder.ts

@@ -10,6 +10,7 @@ import { IDocWorkerMap } from "app/server/lib/DocWorkerMap";
 import { expressWrap } from "app/server/lib/expressWrap";
 import { GristServer } from "app/server/lib/GristServer";
 import { getAssignmentId } from "app/server/lib/idUtils";
+import { addAbortHandler } from "app/server/lib/requestUtils";
 
 /**
  * Forwards all /api/docs/:docId/tables requests to the doc worker handling the :docId document. Makes
@@ -101,7 +102,7 @@ export class DocApiForwarder {
 
     // If the original request is aborted, abort the forwarded request too. (Currently this only
     // affects some export/download requests which can abort long-running work.)
-    req.on('close', () => controller.abort());
+    addAbortHandler(req, res, () => controller.abort());
 
     const options: RequestInit = {
       method: req.method,