performance fix for return edges; if there were N start points (e.g. in a backwards analysis) and we did an unbalanced analysis, then the return-foe function was computed N times; now this is done only once
Merge branch 'feature/performance-fix-for-return-edges' into develop
adding synchronization on "val" due to possible race conditions (thanks to Damien Octeau for reporting this!)
Migrated DEBUG to SLF4J logging