Have a more compact database format

[tdussa]

By default, one database row is stored per event. However, there are many use cases in which a complete list of these events is not desired. In many cases, it is sufficient to know when a given IP/MAC/VLAN/origin/interface combination occured for the first and last times (and possibly how many times it did occur in total). This allows for a much more compact database scheme.

I have added the option to use such a "compact" scheme instead. I'll drop a pull request for this.