QModbusServerPrivate::storeEvent() needs fixing

Currently, the modbus communication event log is filled with QModbusServerPrivate::storeEvent().

Suggestions:

1. rename it to storeCommEvent() to make clear the function stores a communication event in the CommEventLog event buffer

2. We either need to make sure that we fill the event buffer cleanly even in situations where derived classes reimplement QModbusServer::processPrivateModbusRequest() and/or alternatively to also make storeCommEvent() available from within QModbusServer class