Subscription events

A new kind of event has been added. Whenever a WebSocket::subscribe or WebSocket::unsubscribe call is made, or when implicit unsubscription happens (such as when a WebSocket has subscriptions and gets closed in any way), a subscription event will fire with:

• the WebSocket involved
• the topic name
• the new number of subscribers to this topic
• the former number of subscribers to this topic

These events can be used to easily manage external subscriptions such as when using Redis to orchestrate multiple instances of uWS. Whenever a subscription event with new subscription count == 1; you can create the corresponding Redis subscription. Whenever a subscription event with new subscription count == 0; you can free the corresponding Redis subscription.