RequestCaptureStream: dumpDefault, haveNonRawStreams, zero ring after dump

- Add 'dumpDefault' option to also dump captured records on the
  *default* ring (i.e. those without a req_id field). Useful for
  when the app hasn't necessarily been nice passing around `req.log`
  and logging on that.
- Zero out the ring after dumping captured records so don't get
  those records *re-dumped* after a second log.warn/error.
- Perf optimization to not serialize if don't have to.