Fix mergejoinsorter

Fix the merge-join-sort and make it the default sort tool. 

Several interesting, and subtle, issues to be addressed, including an OOM. 

The biggest changes was in the MergeJoinSort itself was to change the Row storage from a Persistit Key to Value, which was required because of encoding issues with the BigDecimal class. This introduced its own set of memory consumption issues. 

Added code to skip duplicate entries when asked. 

Update the tests to work with the changes, add new test for BigDecimal.