Testing with mysql now works. Turns out the unique index
(__retired__, keyvalue) wasn't being created.

I added the call to add_class_key_required_unique_constraint to 
create_class_table_indexes defined in in back_mysql.py.
This now matches the function of create_class_table_indexes
in roundup/backends/rdbms_common.py.

Not sure of the effect of this change on existing trackers.
Still need to investigate.