Add volatile to methodNameMap and chainMap (#34)

Modify MethodUtil#methodNameMap and CtSph#chainMap: add `volatile` to keep in line with others

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

@@ -48,7 +48,7 @@ public class CtSph implements Sph {
      * Same resource({@link ResourceWrapper#equals(Object)}) will share the same
      * {@link ProcessorSlotChain}, no matter in which {@link Context}.
      */
-    private static Map<ResourceWrapper, ProcessorSlotChain> chainMap
+    private static volatile Map<ResourceWrapper, ProcessorSlotChain> chainMap
         = new HashMap<ResourceWrapper, ProcessorSlotChain>();
 
     private static final Object LOCK = new Object();

sentinel-core/src/main/java/com/alibaba/csp/sentinel/util/MethodUtil.java

@@ -29,7 +29,7 @@ import java.util.concurrent.ConcurrentHashMap;
  */
 public final class MethodUtil {
 
-    private static Map<Method, String> methodNameMap = new HashMap<Method, String>();
+    private static volatile Map<Method, String> methodNameMap = new HashMap<Method, String>();
 
     private static final Object LOCK = new Object();