Radix Clustered Hash Join is now parallel.

The radix join adds the following plan operatoins:

- Histogram - calculates occurrences per value in a table
- Histogram 2nd pass
- Radix Cluster - clusters an input table based on a prefix sum list and
  a hash value
- Radix Cluster 2nd Pass
- Prefix Sum - generates a prefix sum array based on the histogram
- Merge Prefix Sum
- Barrier
- Create Radix Table

The script in tools/joiner.rb can be used to generate paralle
radix join plans. In addition there are a few changes to the
interface of the fixed length vector to better handle the
down-casted data access. This includes a inc() and
atomic_inc() method to increment a certain value at poin.

All Column Types can be joined now. The Histogram and Radix Cluster methods
are now templated accordingly.
added workstealing task scheduler and set this scheduler as default
Extended RadixJoin to work with PointerCalculators