More resolution

[Owen Carter]

Add db archive to log at higher resolution, eg every 10s for 3 weeks. Something like

rrdtool.create(
                str(self.db_file),
                "--start", "now",
                "--step", "10",
                "RRA:AVERAGE:0.5:1:181440",   # 3 weeks per 10s
                "RRA:AVERAGE:0.5:6:786240",   # 3 months per minute
                "RRA:AVERAGE:0.5:360:158112",  # 3 years per hour
                *ds_list)

And work out how to dump-restore on existing logs...

[Owen Carter]

Cone in commit 6f7aaa9