[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.

Author: Takuya UESHIN <ueshin@happy-camper.st>

Closes #836 from ueshin/issues/SPARK-1889 and squashes the following commits:

fe1c387 [Takuya UESHIN] Apply splitConjunctivePredicates to join condition while finding join keys.