Ensure HTTP/2 connections are gracefully closed on async cancellation

[karpetrosyan]

Closes #756

Checklist

• [x] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [ ] I've updated the documentation accordingly.

TODO

• [x] Add failing tests
• [x] Gracefully handle cancellation for unstarted http/2 requests
• [x] Update changelog