Rework data structure for Local Networks

[cardigliano]

Local Networks are currently identified by a sequential ID and stored in a statically-allocated array (see local_network_names/local_network_aliases in Ntop). This can create issues when adding new local networks to the configuration and the IDs may change. It is requested to change the data structure to dynamically make room for new networks (e.g. store them in the patricia tree) and keep the network-id association in a db.