Add `safe_update_row` and `safe_delete_row` functions.

[jamesaoverton]

To better support collaborative editing, and prevent one user from accidentally overwriting changes by another user, we should add an API function that accepts a "before" state for the row and ensures that the current state of the row is identical to the "before" state. If they are not identical, then presumably some other user has recently made a change. In this case we will return an Err result with a helpful message. Otherwise we make the change, like any other update_row() call.

pub async fn safe_update_row(
        &self,
        table_name: &str,
        row_number: &u32,
        before_row: &JsonRow,
        after_row: &JsonRow,
    ) -> Result<ValveRow> {
   ...
}

Keep in mind that updates cascade, and can change multiple tables. At least for now, we will not fall down the rabbit hole of locking everything down. Our use cases do not call for high loads of simultaneous changes.

I think we should do the same thing for delete_row(). I don't think we need to do anything for insert_row().