refactor: use the ClusterAdapter class from socket.io-adapter package

The ClusterAdapter class has been moved to [1], so that this adapter
only needs to implement to pub/sub mechanism.

Also, [2] should reduce the number of "timeout reached: only x
responses received out of y" errors, since the fetchSockets() requests
will now succeed even if a server leaves the cluster.

[1]: https://github.com/socketio/socket.io-adapter
[2]: socketio/socket.io-adapter@0e23ff0

Author: darrachequesne