[#645] placement/traverser: Extend Node type with public key

There is a need to process public keys of the placement result.

Implement `Node.PublicKey` method which returns storage node's key announced
in netmap.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/services/object_manager/placement/traverser.go

@@ -122,6 +122,8 @@ func flatNodes(ns []netmap.Nodes) []netmap.Nodes {
 // Node is a descriptor of storage node with information required for intra-container communication.
 type Node struct {
 	addresses network.AddressGroup
+
+	key []byte
 }
 
 // Addresses returns group of network addresses.
@@ -129,6 +131,11 @@ func (x Node) Addresses() network.AddressGroup {
 	return x.addresses
 }
 
+// Key returns public key in a binary format. Should not be mutated.
+func (x Node) Key() []byte {
+	return x.key
+}
+
 // Next returns next unprocessed address of the object placement.
 //
 // Returns nil if no nodes left or traversal operation succeeded.
@@ -157,6 +164,8 @@ func (t *Traverser) Next() []Node {
 			// TODO: log error
 			return nil
 		}
+
+		nodes[i].key = t.vectors[0][i].PublicKey()
 	}
 
 	t.vectors[0] = t.vectors[0][count:]