refactor: import the ClusterAdapter abstract class

Imported from https://github.com/socketio/socket.io-redis-adapter/blob/ef5f0da0b4928fd422afc985aec0e233d34400c0/lib/cluster-adapter.ts

This abstract class is currently used by the sharded Redis adapter. We
import it here because the logic can be reused by the other adapters,
which will then only need to handle the pub/sub mechanism.

Author: darrachequesne