Add support for `SubscriptionListener` to listen to [P][UN]SUBSCRIBE notifications

[mp911de]

Registering a MessageListener with RedisMessageListenerContainer or starting to receive messages from RedisMessageListenerContainer requires awaiting until the actual subscription has been registered with Redis. Since subscribing triggers asynchronous activity, the only means to synchronize are sleep timers.

It would be good to actually consume Redis subscription confirmations to [P][UN]SUBSCRIBE commands through an interface along the lines of:

public interface SubscriptionListener {

	SubscriptionListener EMPTY = new SubscriptionListener() {};

	default void onChannelSubscribed(byte[] channel, long count) {}

	default void onChannelUnsubscribed(byte[] channel, long count) {}

	default void onPatternSubscribed(byte[] pattern, long count) {}

	default void onPatternUnsubscribed(byte[] pattern, long count) {}

}

It should be possible to let a MessageListener class implement SubscriptionListener to bind the actual notifications to the message listener.

[christophstrobl]

closed via #2052