Notification service

A notification service is available allowing a client to be notified when significant changes are made to the database by other clients (and also changes made by itself).

The type of change events are those defined by the underlying DBMS used by CommDb. The specific change events are defined by the enumerators of the RDbNotifier::TEvent.

As is common with this kind of service, the request is an asynchronous request that is initiated by a call to CCommsDatabaseBase::RequestNotification() and remains outstanding until either a change to the database occurs, or it is cancelled by a subsequent call to CCommsDatabaseBase::CancelRequestNotification().