PING responses are not decoded properly if Pub/Sub connection is subscribed

[mp911de]

See #576 (comment).

[mp911de]

In a subscription state, it's not distinguishable (from an ordering perspective) whether a response is related to a sent command or it's a Pub/Sub response. Making that distinction would require response inspection and pre-processing. For now, we deny non-pubsub commands once the connection has entered Pub/Sub state.