Dolt does foreign key checks on update when no foreign key columns have changed

Foreign key checks are expensive, and we currently check them on every row that gets changed in an `UPDATE`, even if no columns with a foreign key constraint have changed. We should fix that.

Start by tracing the UPDATE path and its foreign-key validation checks; the issue does not name a file, test, or entry point. Done means UPDATE skips foreign-key checks when no constrained columns changed, while still checking them when relevant columns are modified.

Written by the indexing model from the issue text.