change custom field index from label to name?

There seem to be reasons to make custom field names exposed to users unique within a custom value group, which is what an existing index does. I'm a bit more concerned about issues referring to a custom field programmatically by its name. Currently, if a label is changed, the name does not change, as it is supposed to be used only internally and needs to remain constant. After the label is changed, a new custom field can be created in the group with the same name as the first one started with, which may cause problems. As well, the name is only 64 characters rather than 255, and I think the label characters are sanitized when creating it, both of which provide ways for the name to be non-unique even if the label is.

This issue may be present in other tables.

The patch creates a new index (which I think is good) and also drops the existing one (probably not such a great idea).