(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/lib/Triggers.ts

@@ -14,7 +14,7 @@ import {makeExceptionalDocSession} from 'app/server/lib/DocSession';
 import log from 'app/server/lib/log';
 import {matchesBaseDomain} from 'app/server/lib/requestUtils';
 import {delayAbort} from 'app/server/lib/serverUtils';
-import {proxyAgent} from 'app/server/utils/ProxyAgent';
+import {proxyAgent} from 'app/server/lib/ProxyAgent';
 import {promisifyAll} from 'bluebird';
 import * as _ from 'lodash';
 import {AbortController, AbortSignal} from 'node-abort-controller';