stream rustdoc html content from S3, use streaming rewriter, stream to client

[syphar]

This will start streaming also rustdoc html files from S3 to the client.

First for simple cases, release specific rustdoc assets, which can directly be streamed.

Then I also added a streaming HTML rewriter.

It's a little more complex than I wanted it to be due to

1. the streaming design of the synchronous lol-html rewriter.
2. the fact that I want to use our rendering threadpool to limit the concurrency for CPU-Bound requests.

Generally the first advantage is more paralelism, since we don't have to load the whole HTML into memory before handling it. Second is that we can (IMO) drop file size limits when the content is streamed.

Downside is that any error while streaming will just lead to a dropped connection on the client side, and not a classic 500. This means we would also not see a sentry error in this case.
Which is why I added error! calls in these cases.

Metrics don't show any rewrite OOMs in the metrics, so I don't think we'll have any production issues here.

[GuillaumeGomez]

Looks all good to me, thanks!