processing the node_history table, but that is *way* too slow to do on
the fly (say, from the web interface) cause of the number of records,
so the summary info is stored in the new node_utilization table. I
generate the summary info as new entries are added to node_history (in
SetNodeHistory) but if that becomes too messy, we can just as easily
shift to processing the table on a nightly basis.

Note that I added a new "inception" date field to the nodes table,
which will get set on new nodes, but for existing nodes I have to
derive it from the first entry in the node_history table, or else the
numbers will not make sense.