Introduce logic to resolve volume replica placement within EC rebalancing. (#6254)

* Rename `command_ec_encode_test.go` to `command_ec_common_test.go`.

All tests defined in this file are now for `command_ec_common.go`.

* Minor code cleanups.

- Fix broken `ec.balance` test.
- Rework integer ceiling division to not use floats, which can introduce precision errors.

* Introduce logic to resolve volume replica placement within EC rebalancing.

This will be used to make rebalancing logic topology-aware.

* Give shell.EcNode.dc a dedicated DataCenterId type.

weed/storage/super_block/replica_placement.go

@@ -45,6 +45,15 @@ func NewReplicaPlacementFromByte(b byte) (*ReplicaPlacement, error) {
 	return NewReplicaPlacementFromString(fmt.Sprintf("%03d", b))
 }
 
+func (a *ReplicaPlacement) Equals(b *ReplicaPlacement) bool {
+	if a == nil || b == nil {
+		return false
+	}
+	return (a.SameRackCount == b.SameRackCount &&
+		a.DiffRackCount == b.DiffRackCount &&
+		a.DiffDataCenterCount == b.DiffDataCenterCount)
+}
+
 func (rp *ReplicaPlacement) Byte() byte {
 	if rp == nil {
 		return 0