Add summary node utilization stats. The initial values are derived by
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.
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