Pruning of outdated JDK 6/7 references (plus related polishing)

(cherry picked from commit b325c74216)

spring-websocket/src/main/java/org/springframework/web/socket/handler/ConcurrentWebSocketSessionDecorator.java

@@ -156,7 +156,7 @@ public class ConcurrentWebSocketSessionDecorator extends WebSocketSessionDecorat
 			}
 			finally {
 				this.sendStartTime = 0;
-				flushLock.unlock();
+				this.flushLock.unlock();
 			}
 			return true;
 		}