Modify CfgMgr to read subnets from different sources
Right now CfgMgr reads subnet configuration only from the configuration file. After #3581 and #3582, it will be possible to read it from databases as well.
It is not yet decided whether this will be done every time during packet processing (faster startup, slower processing) or during reconfiguration (slower startup, faster processing). The decision will be made as part of the #3579 ticket.