The `Issuable::DestroyService` gets called for
`DELETE /api/projects/:id/merge_requests/:merge_request_iid`
endpoint.

When deleting an issuable (`Issue` or `MergeRequest`), the
associated Todo records need to be deleted as well. Before this
change, it was calling `Todo#destroy` on each associated todo that
results to N+1 issues.

Deletion of todos is now being done asynchronously since it's
possible that the deleted issuable has lots of associated todos
and doing it synchronously can take time. This also makes it
consistent with how we delete todos in other cases (other services
can be found under `app/services/todos/destroy/` directory).

Deletion is being done in batches to avoid statement timeouts in
case there are a lot of todos to be deleted.