🐛 Bug Report — Runtime APIs: successive `fetch()`es fail if upstream doesn't consume body

Hey! 👋 When investigating cloudflare/workers-sdk#5095, we observed `Network connection lost` errors from successive calls to `fetch()` if the upstream didn't consume the request body. Specifically, every other request would fail. See https://github.com/mrbbot/workerd-network-connection-lost-repro for a minimal reproduction. Uncommenting this line https://github.com/mrbbot/workerd-network-connection-lost-repro/blob/04b2a464c8c269fb467ca5247efbb6da2303c9f5/user.capnp#L19 "fixes" the issue. It also seems like this line https://github.com/mrbbot/workerd-network-connection-lost-repro/blob/04b2a464c8c269fb467ca5247efbb6da2303c9f5/index.mjs#L32 needs to create a body with at least `2**16 + 1` bytes to trigger a reproduction.

This issue feels related to https://github.com/cloudflare/workerd/issues/1376 and https://github.com/cloudflare/workerd/issues/960. In https://github.com/cloudflare/workerd/issues/960#issuecomment-1663042638, it was suggested that `workerd` should be canceling the upload in cases where a response is returned without reading the body. It feels like the same thing should be happening here.

/cc @petebacondarwin @RamIdeas