Support upsert conflicts for non-unique columns

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From docs (https://www.sqlite.org/lang_upsert.html):

The UPSERT processing happens only for uniqueness constraints. A "uniqueness constraint" is an explicit UNIQUE or PRIMARY KEY constraint within the CREATE TABLE statement, or a [unique index](https://www.sqlite.org/lang_createindex.html#uniqueidx).

I wonder if we should also support it for other columns. This would then be executed by the server, since SQLite doesn’t support it. Basically if insert causes an error, run:

UPDATE "users"
SET name = 'Marc'
WHERE name == 'John'

(Must be one transaction)