Remove UNIQUE for slug in wp_terms

[nacin]

To set us up for future changes to the taxonomy API, we should remove the UNIQUE key for 'slug' for wp_terms. Said future changes include:

• Splitting shared terms on update (#5809)
• Stop creating shared terms (#21950)
• Forcibly split remaining shared terms
• Merge wp_terms and wp_term_taxonomy (I can dream, right?)

The term_exists() check should prevent duplicate terms from ever being inserted (before, of course, we fix #5809 and #21950). But, this needs unit tests, particularly because there is a case where term_exists() breaks. See #17689, which blocks this ticket.

For the version 3.5 we would need:

• Remove the UNIQUE from slug on wp_terms
• Improvments on term_exists()

Just checking so we can delivery something on 3.5

Is this part of 3.5 or 3.6?

No unit tests; #17689 not fixed yet. Punting.

Yeah, I thought this was punted a while ago.