[SPARK-1889] [SQL] Apply splitConjunctivePredicates to join condition while finding join ke...

...ys.

When tables are equi-joined by multiple-keys `HashJoin` should be used, but `CartesianProduct` and then `Filter` are used.
The join keys are paired by `And` expression so we need to apply `splitConjunctivePredicates` to join condition while finding join keys.