Delete unreferenced derivations in batches

To avoid a long blocking query.

guix-data-service/data-deletion.scm

@@ -425,15 +425,25 @@ WHERE build_server_id = $1
        build-server-ids)))
 
   (define (delete-unreferenced-derivations-source-files conn)
+    (define (delete-batch)
       (exec-query
        conn
        "
 DELETE FROM derivation_source_files
-WHERE NOT EXISTS (
+WHERE id IN (
+  SELECT id
+  FROM derivation_source_files
+  WHERE NOT EXISTS (
     SELECT 1
     FROM derivation_sources
     WHERE derivation_source_file_id = derivation_source_files.id
-)"))
+  )
+  LIMIT 100
+)
+RETURNING id"))
+
+    (while (not (null? (delete-batch)))
+      #t))
 
   (define (maybe-delete-derivation conn id)
     (match (map