Jobsets: add a SERIAL, unique, non-null id column

A postgresql column which is non-null and unique is treated with
the same optimisations as a primary key, so we have no need to
try and recreate the `id` as the primary key.

No read paths are impacted by this change, and the database will
automatically create an ID for each insert. Thus, no code needs to
change.

src/sql/upgrade-59.sql

@@ -0,0 +1,4 @@
+-- will automatically add unique IDs to Jobsets.
+ALTER TABLE Jobsets
+  ADD COLUMN id SERIAL NOT NULL,
+  ADD CONSTRAINT Jobsets_id_unique UNIQUE (id);