Trace:
- move from pipelet.js to trace.js, envision extraction to separate npm module
- Rename Trace into Trace_Domain

- trace():
  - query_add(): use Trace_Domain.default_trace_level instead of 5 for default level
  - use for..in instead extend_2() to merge trace object properties into _trace

- debugging:
  - add embeded tests for debugging, preparing automated tests
  - add console.log() traces for debugging
  - fix bugs:
    - trace():
      - fix trailer attribute name
      - after emit(), call trace.reset instead of this.reset()
    - no_trace_for_instance(): replace _trace._level < level by level < _trace._level
    - trace.reset(): do not initialize flow and realm in _trace object
    - trace.query_default(): use Trace_Domain prefix for class attributes