Cloudflare alt-svc onions cause a different exit to be used at each request

For some websites, such as zerobin.net which use Cloudflare's new onions, each request might cause a different exit to be used. This is bad both from a load perspective as well as a fingerprinting perspective.

Steps to reproduce: Go to zerobin.net and create a new paste with "allow discussion" enabled. Post several comments and observe that the visual hash of the IP often changes with each comment. Check the circuit display to verify.