backend: use swh.scheduler to dispatch cooking tasks

db: remove task_uuid field backend: use swh.scheduler to dispatch cooking tasks

---

Migrated from D278 (view on Phabricator)

sql/swh-vault-schema.sql

@@ -22,9 +22,8 @@ create table vault_bundle (
   type cook_type not null,  -- requested cooking type
   object_id obj_hash not null,  -- requested object ID
 
-  task_uuid uuid not null,  -- celery UUID of the cooking task
+  task_id integer,  -- scheduler task id
   task_status cook_status not null default 'new',  -- status of the task
-
   sticky boolean not null default false, -- bundle cannot expire
 
   ts_created timestamptz not null default now(),  -- timestamp of creation