Implement MySQL logger, refactor some events and hook the loggers to aggregator and aggregator to events (OMS-372)
Using DB-agnostic implementation for easier testing (OMS-372)
Fixes of issues found during testing + improved generalized implementation of SQL DB user stats logger (OMS-372)
Merge pull request #64 from opennode/oms-372

Add accounting log for the aggregated user infrastructure (OMS-372)
Clarify log message + cleanup (OMS-372)