R1:c7dc59f9c403

Don't call flush() when emitting responses

Summary: Fixes T7620. I don't fully understand exactly what's going on here, but we don't actually need to call `flush()`.

Test Plan:
- Put timing code around the `echo`.
- Made a fake page that emitted a lot of data.
- Saw the `echo` block proportionate to data size under `curl --limit-rate ...`.
- See T7620.
- Downloaded a large file, got a reasonable progress bar and no obvious memory use issues.

Reviewers: btrahan

Reviewed By: btrahan…