Migrate {taxonomy_index} table: table now has a unique primary key

Problem

Migrate follow up for #610076: Add a primary key to the {taxonomy_index} table and prevent duplicates being inserted.

In D8, the taxonomy_index table has a primary key. In D7 this primary key does not (yet) exist, so there is a possibility that duplicate records will be migrated and the migration should take this into account. The chance on duplicate records is not only theoretical, it actually occurs in real life, a.o. for multilingual nodes (using entity translation), where different languages refer to the same terms. but if the old way of translating (translation sets) is migrated as well, this will also occur when processing the nodes belonging to the same translation set.

If the parent issue gets committed to D7 as well, this remains a problem for nodes belonging to the same translation sets.

Remaining tasks

I am not sure that the migration for this table needs to be changed, but as per https://groups.drupal.org/node/402803, I am filing an issue here anyway. So decide if and how to adapt the migrate issue.