cleanup dataprocessor code

[Dieterbe]

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[replay]

I guess the string DP is for DataProcessing? I don't think that's obvious, and f.e. the stuff in /input could also be considered "data processing". I think it would be nice to have some standard defining what our log prefix should be, but anyway for now this PR is still an improvement to master

[replay]

Side note: I've noticed that in Cortex they're using a logger which automatically adds a field to every log statement which indicates where this log line has been produced. I think that's a really nice solution. Logrus could do that too with log.SetReportCaller(true), this would make our log statement prefixes redundant and we could remove them