(core) Add logging of errors whenever ProxyAgent is used, and a test for it.

Summary:
Also:
- Move ProxyAgent to from app/server/utils to app/server/lib, which is
  the more usual place for such classes.
- Refactor a helper (delayAbort) that node was reporting a leak in.

Test Plan: Added a test case, and tested manually.

Reviewers: JakubSerafin

Reviewed By: JakubSerafin

Subscribers: JakubSerafin, paulfitz

Differential Revision: https://phab.getgrist.com/D3897

app/server/utils/ProxyAgent.ts

@@ -1,11 +0,0 @@
-import {HttpsProxyAgent} from "https-proxy-agent";
-import {HttpProxyAgent} from "http-proxy-agent";
-
-export function proxyAgent(requestUrl: URL): HttpProxyAgent | HttpsProxyAgent | undefined {
-    const proxy = process.env.GRIST_HTTPS_PROXY;
-    if (!proxy) {
-        return undefined;
-    }
-    const ProxyAgent = requestUrl.protocol === "https:" ? HttpsProxyAgent : HttpProxyAgent;
-    return new ProxyAgent(proxy);
-}