logilab-database #6662663 implicit naming of unique constraint [in-progress]

currenly, implementation of create_index with unique=True doesn't name the index, while it's used in drop_index. Hence this relies on backend implementation detail, which is no good.

appeared in<not specified>
done in1.16.0
closed by<not specified>
patchUse index name on unique index creation. Closes #6662663 [reviewed]