Fix various issues with fallback directory data handling

Fix various issues with fallback directory data handling

* support maximum history age in _avg_generic_history()
* fix division-by-zero trap in _avg_generic_history()
* skip missing (i.e. null/None) intervals in _avg_generic_history()
* Python timedelta.total_seconds() function not available in 2.6;
  replace with equivalent expression
* set DEBUG logging level to make relay exclusion reasons visible
* move CUTOFF_GUARD test to end in order to expose more exclusion
  reasons

Patch by "starlight", merge modifications by "teor".