Uploads are not cancelled when a New Identity is requested

From legacy/trac#24421 (moved):

1. Set High security setting (I was using that one, didn't bother to see if it affects other security settings).
2. Go to the Bezos Washington Post Secure Drop: jcw5q6uyjioupxcc.onion (you can verify it here: securedrop.org/directory )
3. Click on "Submit Documents".
4. Click on "Use New Codename".
5. Select some large file (we don't want the upload to finish so we don't waste the journalists' time wasting Bezos' money) like a Tor Browser tar.gz
6. Open your Gnome System Monitor and go to "Resources" and watch the network graph. You should see the upload at its peek values.
7. Close the tab, you can see that the upload is still going.
8. Click on New Identity, again, upload is still going.
9. Close the Tor Browser, upload halts.

It works for clearnet websites as well, you can test with ​https://catbox.moe/ following essentially the same procedure as the last ones above (i.e. 5-9).

This issue is observable with Tor Browser 6.5.2 as well.