reset the QElapsedTimer in usecTimestampNow() periodically to prevent error

We've been seeing reports of "unreasonable flight times" - these appear to happen when an Assignment Client has been running for a "long time"... I believe this issue is caused by a drift/roll-over error in QElapsedTimer. This change will reset the QElapsedTimer every hour and move the TIME_REFERENCE ahead accordingly to keep the usecTimestampNow in sync.