Describe the bug
当多个线程同时向同一服务注册同一集群时，该集群会执行多个健康检查任务。com.alibaba.nacos.naming.core.Cluster类代码

很明显，该init方法不是原子操作，存在多个线程同时读取inited变量为false的情况，则会将同一健康检查任务执行多次。

Expected behavior
同一集群实例，只应该初始化一次

Acutally behavior
同一集群实例，可能初始化多次

How to Reproduce
Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Desktop (please complete the following information):

• OS: [e.g. Centos]
• Version [e.g. nacos-server 1.3.1, nacos-client 1.3.1]
• Module [e.g. naming/config]
• SDK [e.g. original, spring-cloud-alibaba-nacos, dubbo]

Additional context
Add any other context about the problem here.