Enforce ORDER BY and LIMIT clauses in `term_exists()` queries.

In case of edge cases where a duplicate term might exist in a replicated
database for a split second, these explicit query clauses ensure that
`term_exists()` will always recognize the oldest matched term as the
canonical one. See [30238] for background.

Props pento.
See #22023, #5809.
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Do not create shared taxonomy terms.

A "shared" term occurs when two entries in the `wp_term_taxonomy` table share a
single `term_id`, and thereby correspond to the same row in `wp_terms`. This
changeset stops the practice of creating shared terms: each new row in
`wp_term_taxonomy` will receive its own row in `wp_terms`. The new strategy
for term creation depends on whether the installation's database schema is up
to date for 4.1:

* If so, terms are allowed to be created with the same slug as an existing term, as long as they are in different taxonomies and do not share a parent. Thus, a new tag with the slug 'wordpress' can exist alongside a category with the slug 'wordpress'.
* If not, new terms will be forced to have unique slugs. Thus, on an installation containing a category with the slug 'wordpress', a new tag 'WordPress' will get the slug 'wordpress-2'.

Fixes #21950. See #5809.
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