Fix deadlock bug in Env static initialization (#610)

Signed-off-by: Carpenter Lee <hooleeucas@163.com>

sentinel-core/src/main/java/com/alibaba/csp/sentinel/Constants.java

@@ -41,7 +41,7 @@ public final class Constants {
     public final static String TOTAL_IN_RESOURCE_NAME = "__total_inbound_traffic__";
 
     public final static DefaultNode ROOT = new EntranceNode(new StringResourceWrapper(ROOT_ID, EntryType.IN),
-        Env.nodeBuilder.buildClusterNode());
+        new ClusterNode());
 
     /**
      * Statistics for {@link SystemRule} checking.

sentinel-core/src/main/java/com/alibaba/csp/sentinel/Env.java

@@ -20,6 +20,13 @@ import com.alibaba.csp.sentinel.node.DefaultNodeBuilder;
 import com.alibaba.csp.sentinel.node.NodeBuilder;
 
 /**
+ * Sentinel Env. This class will trigger all initialization for Sentinel.
+ *
+ * <p>
+ * NOTE: to prevent deadlocks, other classes' static code block or static field should
+ * NEVER refer to this class.
+ * </p>
+ *
  * @author jialiang.linjl
  */
 public class Env {