[SPARK-10443] [SQL] Refactor SortMergeOuterJoin to reduce duplication

`LeftOutputIterator` and `RightOutputIterator` are symmetrically identical and can share a lot of code. If someone makes a change in one but forgets to do the same thing in the other we'll end up with inconsistent behavior. This patch also adds inline comments to clarify the intention of the code.

Author: Andrew Or <andrew@databricks.com>

Closes #8596 from andrewor14/smoj-cleanup.