MigrationType relies on genID (https://github.com/mongodb/mongo/blob/master/src/mongo/db/s/balancer/type_migration.h#L89) in its getName function. Remove any uses of genID, and thus getName, in favor or querying on the ns_1_min_1 index. 

According to kaloian.manassiev here (https://mongodbcr.appspot.com/496100032/diff/476240012/src/mongo/db/s/balancer/type_migration.h#newcode89), this is only relevant to duplicate key handling.