decision: fix design to do partial explorations

* make findSplitterRec and all related helper functions' return
  type trivalued, to be able to distinguish between
  "partial exploration" vs "done exploration but found nothing"

* keep additional data structure to remember to what extent the
  partial exploration has been completed so not to repeat it. we
  can use this to make multiple passes on formula with arbritrary
  order of thresholds for exploration