discovery

Discovery:
1. if the cluster is not full: add self to the cluster and return all the peers.
2. if the cluster is full, return a fullcluster error.
They will not. Machines will register themselves into the cluster. Then they sorted the existing machines in the cluster. Next it will find where is itself. If it is within the clusterSize, return nil or return full. 

The check will not depend on the number of registered machines in the cluster dir. Line35 is only a fast checking path.