Revert "fix(node/http): do not buffer first chunk (#2989)"

[bartlomieju]

This reverts commit eca4dbf.

This commit caused performance regression in express server from
25k rps to 20k rps.

CC @kt3k

[ry]

LGTM

Can you show how to reproduce this bug so we can re-introduce the fix, but without the regression?

[kt3k]

The below server responds immediately with chunk "A", and then "B" 1 sec later with the fix, but it responds "AB" after 1 sec without the fix.

import * as http from "./node/http.ts";
http.createServer((_, res) => {
  res.write("A");
  setTimeout(() => {
    res.write("B");
    res.end();
  }, 1000)
}).listen(3000);