correct output fields in multi-entity queries

* in KWS, the fields are now identified per lookup and not by PK as earlier
  - as a consequence file,run,lumi will get it's proper fields
* keylearning is indexing the field as earlier per (sys, urn)
  - we may add additional param lookup into db, but I see no need...

 Note: in far future we may think if we want to allow an additional join (e.g. by run, lumi, file on it's PK to be processed by user himself), but lets say it's not allowed for now
(P.S.2. in KWS the  multi-entry queries are enumerated,  but still none keywords are matched to multi-entity lookup)
pep8 fixes for keylearning
Merge pull request #4098 from dmwm/kws_cleanup_pylint_lookup

better handling of output fields for multi-entity queries (apply only after KWS pep8 fixes pull)