small optimizations for streaming html rewrite

[syphar]

1. don't use the std sync channels in this async / sync mix. Tokio's channels can handle the mix better.
2. use bounded channels so in the case that (for example) the sender is too fast / the receiver to slow, we won't end up in OOM land.
3. add Send + 'static bounds so I don't do stupid stuff any more (like ensure current tracing span is passed into streaming html rewrite #2874, fix in ensure current tracing span is passed into streaming html rewrite #2974)
4. errors: try to preserve the context / error chain when we report errors by using the anyhow context, and also our report_error util for sentry.

[GuillaumeGomez]

Thanks!