Fix issue #642 NPE on Join over Salted table

Also included 

Bug fixes:
1. Wrong calculation of number of Longs in ValueBitSet#toBytes().
2. Errors in table self join.
3. Redundant and mistaken calling to optimize() for join queries.

Join runtime code refactoring.
Add interface Tuple#getValue(cf, cq, ptr) to avoid creating new KeyValues in joins.
Skip-merge optimization for joins.