fix: delete duplicate client instances

- Due to applying a primary key constraint on app_name, instance_id,
  active installations with client_instances that have already violated
  this unique constraint will fail to migrate.
- This commit drops all but the latest instance of each (app_name,
  instance_id) tuple, thus preparing the database for applying the
  unique constraint

Co-authored-by: Ivar Conradi Osthus <ivarconr@gmail.com>
Co-authored-by: Fredrik Strand Oseberg <fredrik.no@gmail.com>

src/migrations/20210302080040-add-pk-to-client-instances.js

@@ -3,6 +3,7 @@
 exports.up = function(db, cb) {
     db.runSql(
         `
+    DELETE FROM client_instances a USING client_instances b WHERE a.app_name = b.app_name AND a.instance_id = b.instance_id AND a.created_at < b.created_at;
     ALTER TABLE client_instances ADD PRIMARY KEY (app_name, instance_id);
   `,
         cb,
@@ -13,7 +14,7 @@ exports.down = function(db, cb) {
     db.runSql(
         `
         ALTER TABLE client_instances DROP CONSTRAINT client_instances_pkey;
-        DROP INDEX client_instance_pkey
+        DROP INDEX IF EXISTS client_instance_pkey
     `,
         cb,
     );