Jobs: add a nullable jobset_id foreign key to Jobsets.

Also, adds an explicitly named "jobs" accessor to the Jobsets
Schema object, which uses the project/jobset name.

src/sql/hydra.sql

@@ -146,9 +146,11 @@ create table JobsetInputAlts (
 create table Jobs (
     project       text not null,
     jobset        text not null,
+    jobset_id     integer null,
     name          text not null,
 
     primary key   (project, jobset, name),
+    foreign key   (jobset_id) references Jobsets(id) on delete cascade,
     foreign key   (project) references Projects(name) on delete cascade on update cascade,
     foreign key   (project, jobset) references Jobsets(project, name) on delete cascade on update cascade
 );

src/sql/upgrade-60.sql

@@ -0,0 +1,10 @@
+-- Add the jobset_id columns to the Jobs table. This will go
+-- quickly, since the field is nullable. Note this is just part one of
+-- this migration. Future steps involve a piecemeal backfilling, and
+-- then making the column non-null.
+
+ALTER TABLE Jobs
+  ADD COLUMN jobset_id integer NULL,
+  ADD FOREIGN KEY (jobset_id)
+      REFERENCES Jobsets(id)
+      ON DELETE CASCADE;