HashMap init optimize when adding new ClusterNode to cluster node map (#465)

- reuse old map size to avoid rearrange and rehash the map

sentinel-core/src/main/java/com/alibaba/csp/sentinel/slots/clusterbuilder/ClusterBuilderSlot.java

@@ -81,7 +81,7 @@ public class ClusterBuilderSlot extends AbstractLinkedProcessorSlot<DefaultNode>
                 if (clusterNode == null) {
                     // Create the cluster node.
                     clusterNode = Env.nodeBuilder.buildClusterNode();
-                    HashMap<ResourceWrapper, ClusterNode> newMap = new HashMap<ResourceWrapper, ClusterNode>(16);
+                    HashMap<ResourceWrapper, ClusterNode> newMap = new HashMap<ResourceWrapper, ClusterNode>(Math.max(clusterNodeMap.size(), 16));
                     newMap.putAll(clusterNodeMap);
                     newMap.put(node.getId(), clusterNode);