from peliodic sending via statistics channel
 to passive retrieval via config session

- Added "getstats" command to retrieve statistics data via config session.
  = type "Auth getstats" command using bindctl shows statistics data.
- Removed StatisticsIntervalTimer
- Removed "sendstats" command
- Changed that each config command can return values.
  - returns Element::createlist(), Element::fromJSON("{}") or Statistics

Opinion:
- Each command should return conditions and values using createAnswer()
   rather than using throw/catch.