Use insert on conflict for url shortener example

fixes #290

examples/url_shortener/main.go

@@ -28,19 +28,9 @@ func afterConnect(conn *pgx.Conn) (err error) {
 		return
 	}
 
-	// There technically is a small race condition in doing an upsert with a CTE
-	// where one of two simultaneous requests to the shortened URL would fail
-	// with a unique index violation. As the point of this demo is pgx usage and
-	// not how to perfectly upsert in PostgreSQL it is deemed acceptable.
 	_, err = conn.Prepare("putUrl", `
-    with upsert as (
-      update shortened_urls
-      set url=$2
-      where id=$1
-      returning *
-    )
-    insert into shortened_urls(id, url)
-    select $1, $2 where not exists(select 1 from upsert)
+    insert into shortened_urls(id, url) values ($1, $2)
+    on conflict (id) do update set url=excluded.url
   `)
 	return
 }