fix for clmn not found bug

Nested outer joins could result in a column not found error thrown by the ExpressionAssembler in the compiler. The error occurred when a MapJoinProject was inserted, it had to shield the columns from subqueries. The proposed solution tracks whether the MapJoinProject visitor is visiting a node within a subquery, if so it does not add its column.

The commented out test case appears to be a different bug as described in: https://trello.com/c/CH388bkQ/837-lost-track-of-column-when-a-nested-subquery