Polish: de-duplicate helper statistics objects

Polish: de-duplicate helper statistics objects

* Combine the stats structure on per-helper server classes. For more
  consistent statistic gathering.

* Add initStats() method to initialize statistics variables correctly.
  Previously only done for some counters on stateless helper objects.

* Add missing accounting of pending lookups in stateful helper code.

* Add counter for replies received from the helper.

* Add reporting of replies received back from each helper.

There are no logic or decision making logics affected by these changes.
The new increment/decrement and stats are purely affecting statistical
report outputs.

FUTURE TODO:
* replace the 'busy' flag on stateful helpers with pending>0 check
  as used by stateless helpers to indicate queue count.