Don't recycle a join when joining for a different property.

in Genome, CQID is failing to find existing SomVal models when searching on tumor_sample.id and normal_sample.id.  This turns out to be because the same join alias is used for both since both join to the same column of the subject table.  This query never returns a result since the same row cannot have two different values for this column.  Here's a new test that demonstrates the failure and a patch to QueryPlan that makes it pass.