Redesign DataNode volumeMap to include all types of Replicas

With the new append design, replicas at a DataNode could be in one of the five states: Finalized, ReplicaBeingWritten, ReplicaWaitingToBeRecovered, ReplicaUnderRecovery, and Temporay. This jira is to
1. define the in-memory representation of these five types of replicas
2. redefine volumeMap to include all five types of replicas and remove ongoingCreates map.