Dispatch events when channel subscriptions are created and destroyed.

[joekaram]

This PR adds two events:

1. User subscribing to channel
2. User unsubscribing from channel

[taylorotwell]

Please provide an explanation of what this feature could be used for.

[joekaram]

I have a use case, discussion, in which I would like to have the channels that have active connections, accessible from other processes.

I tried creating a Redis channel manager, that implements ChannelManager and ChannelConnectionManager contracts:

• Implementing ChannelManager was complicated as the Redis client is executing asynchronously and returning promises. Using React\Async\await(...) breaks on the destructor when Reverb dispatches other jobs (e.g: PruneStaleConnections) due to the error Cannot switch fibers in destructor..
• Implementing ChannelConnectionManager wouldn't work cause connections can't be serialized and stored in Redis.

Therefore, I added the two events so I can listen to them and manage a copy of those connections, outside of the framework, to serve my purpose.

If there's any other suggestion or way to achieve this, would really appreciate it!!