Add `LogHandler`. Migrate to the new log format.

* `LogHandler` logs the result of an HTTP request, wrapping a
  `ResponseWriter` in a `logResponseWriter` to record the response
  time, status code, and content length.
* Tag all incoming requests with the `X-Request-Id` header. This is a
  convention used by Heroku and restify, and allows correlating log
  lines for the same request.
* Add `Worker.id`. Tag all `Worker.logger.*` calls with the ID.
* Remove `log.Printf` calls from `Worker.sniffer()`.
* Remove the deprecated `debug.PrintStack()` call from
  `Handler.PushSocketHandler()`.
* Replace `BadPayloadErr` with `CodeBadPayload`. Remove unused errors.