Add a table to record where narinfo files were fetched from

Otherwise it's hard to associated narinfo files to build servers.

sqitch/verify/narinfo_fetch_record.sql

@@ -0,0 +1,7 @@
+-- Verify guix-data-service:narinfo_fetch_record on pg
+
+BEGIN;
+
+-- XXX Add verifications here.
+
+ROLLBACK;