1. nats subscription draining is removed, since it is asynchronous,
and there is no reliable way to detect when it is finished.
Remove this option to avoid confusion.
2. nats connection draining is kept, and use 2 callbacks to detect
draining timeout (timeout is set via `nats.Options`) or finish.
3. Also honour options passed in `broker.Init()` (previously only
`broker.New()` is honoured).
