* shell: volume.tier.move can move volumes between data centers
-fromDataCenter scopes volume selection to volumes with a replica in
that data center. -toDataCenter constrains move destinations and
replication fulfillment. With identical disk types both flags are
required, moving full volumes between data centers on the same tier.
* shell: assert node identity in data center filter test
* shell: tier move resumes when the volume is already on the target
A replica already on the target tier and data center, typically left by
an interrupted earlier run, anchors the move: skip the copy and only
complete replication fulfillment and old replica cleanup. Previously
such volumes hit the no-destination path and the stale source replicas
were never removed.
