jobqueue: store an expiry date

This introduces an expiry date (default: 14 days from insert date) and
adjust the service-maintenance script to delete jobs that are older than
the expiration date.

pkg/jobqueue/dbjobqueue/schemas/005_jobs_expiry.sql

@@ -0,0 +1,16 @@
+-- add the expires_at column
+ALTER TABLE jobs
+ADD COLUMN expires_at timestamp DEFAULT NOW() + interval '14 days';
+
+-- We added a column, thus we have to recreate the view.
+CREATE OR REPLACE VIEW ready_jobs AS
+SELECT *
+FROM jobs
+WHERE started_at IS NULL
+  AND canceled = FALSE
+  AND id NOT IN (
+    SELECT job_id
+    FROM job_dependencies JOIN jobs ON dependency_id = id
+    WHERE finished_at IS NULL
+)
+ORDER BY queued_at ASC