1093870 - Use far less RAM during publish.

It was discovered that
pulp.server.managers.repo.unit_association_query.RepoUnitAssociationQueryManager._merged_units_unique_units()
stored every unit it iterated over on its associations_lookup parameter. This led to huge amounts of RAM
being used during Yum publish operations[0], and potentially in other operations as well. This commit
ensures that the method does not alter either of its two parameters.

[0] https://bugzilla.redhat.com/show_bug.cgi?id=1093870