nacos-common-1.3+.jar ConcurrentHashSet.java is not compatible with jdk1.6 or 1.7

[wanshiliuyun]

Describe the bug
below exception throwed when run nacos-client on jdk 1.6 or 1.7

java.lang.NoSuchMethodError: java.util.concurrent.ConcurrentHashMap.keySet()Ljava/util/concurrent/ConcurrentHashMap$KeySetView;
at com.alibaba.nacos.common.utils.ConcurrentHashSet.iterator(ConcurrentHashSet.java:49)
at com.alibaba.nacos.common.notify.DefaultPublisher.receiveEvent(DefaultPublisher.java:176)
at com.alibaba.nacos.common.notify.DefaultPublisher.openEventHandler(DefaultPublisher.java:117)
at com.alibaba.nacos.common.notify.DefaultPublisher.run(DefaultPublisher.java:94)

use JDK8 compile to target 1.6 is not compatible with jdk1.6 or 1.7

Expected behavior
no exception

Acutally behavior
throw exception

How to Reproduce
Steps to reproduce the behavior:

1. run new DefaultPublisher().run on jre 1.6 or 1.7

Desktop (please complete the following information):

• OS: [All]
• Version [nacos-common1.3.1, nacos-common1.4.1]
• Module [common]