nexus-writer: go back in time and get f144 update previous to run #424
Description
This is a follow-on to #423 as you need to know which PVs are interesting and want to end up with values in selog.
If I have a PV which has updated before the run started and didn't update during the run, I would still like to log its most recent value and place that in selog - the ICP does this by going back in the database of sample env logs. We would need to go back in the _sampleEnv topic and search for the most recent update with a key of the PV name.