Implemented new bulk event insert commit feature.

The mentat-storage.py daemon has now ability to use transaction savepoints to reduce number of commits and thus greatly enhance event storage rate. The feature is configurable and optional. The mentat.services.eventstorage needed to be upgraded to support this new feature. Also, the pyzenkit module contained a bug, which prevented this feature to work correctly, so it needed to be fixed as well. (Redmine issue: #4572)