DatanodeAdminMonitor should track under replicated containers and complete the admin workflow accordingly

HDDS-2459 allowed the replicationManager to take care of containers which are under-replicated due to decommission and maintenance.

Its also exposed a new API to return a ContainerReplicaCount object:

getContainerReplicaCount(Container container)

This object will allow the DatanodeAdminMonitor to check if each container is "sufficiently replicated" before decommission or maintenance can complete and hence can be used to track the progress of each node as it progresses though the admin workflow.

We should track the containers on each node in administration and ensure that each is closed and sufficiently replicated before allowing decommission to complete.