Refactored / bugfixed / delta-enabled radix join

- RadixJoin did not handle vertical as well as horizontal offset into
  containers correctly but mostly assumed too simplistic structures
- Instead of just considering the main attribute vector, the join will
  now consider both attribute vectors and handles the delta correctly
- Inputs to the join may need to be validated ("ValidatePositions") to
  provide a consistent view on inputs that are subject to concurrent
  modifications
- RadixCluser and histogram showed a lot of code duplication. This has
  been refactored
Merge pull request #334 from kaihowl/pullrequest/radixjoin

Refactored / bugfixed / delta-enabled radix join