fix: make req.respond() exclusive

[keroxp]

• BREAKING CHANGE: req.writeTrailer() now remove.
  • use trailers options in ServerResponse
• fix: ServerRequest.respond() is now exclusive.

---

Now code like below are valid handler for ServeHandler but is not recommended.

listenAndServe({...}, async req => {
  // await is optional within handler
  req.respond({status: 200, body: "ok"})
});