High-res logging tool & minor fixes

• We now have a logging tool that uses the firehose stream to create an exhaustive log of all measurements. For example, to record everything and write to log.db, use something like this:

$ ./bin/sdm630_logger -v record -f log.db

• We also log voltage total harmonic distortion (THD). Please note that these values seem to fluctuate - you need to judge the data quality for yourself.

• The aggregated import and export kWh values are now queried correctly from the smaller SDM devices.