policy trace-logging improvements

The logs can be triggered from policy actions, in per-request fashion:
- they're on LOG_DEBUG level but always sent, regardless of log config
- those messages will show double group tags: "[reqdbg][foo   ]"
  (but they lack proper meta-data - about location of the log's origin)
- reqdbg is *in addition* to normal logs, so the lines may be duplicated
  if that's how the logging was configured