Record job success without deleting the job record

Previously, the records for jobs would be deleted. It's useful to know when
jobs were inserted in to the database, as well as when they succeeded (if they
have). This change also makes it possible to keep track of jobs that have
failed, as they won't be deleted.

sqitch/verify/dates_to_load_new_guix_revision_jobs.sql

@@ -0,0 +1,8 @@
+-- Verify guix-data-service:dates_to_load_new_guix_revision_jobs on pg
+
+BEGIN;
+
+SELECT created_at FROM load_new_guix_revision_jobs WHERE FALSE;
+SELECT succeeded_at FROM load_new_guix_revision_jobs WHERE FALSE;
+
+ROLLBACK;