是否可以提供支持自定义SC的自身实例TTL的能力

[colin-si]

Is your feature request related to a problem? Please describe.
当前SC自身实例的TTL周期固定为30*（3+1），是否可以将该配置开放出去，支持可以根据自身系统的具体情况进行配置。比如说，当前实例下线时间配置成了2min，当SC发生重启时，有可能在2min内会查询到多个SC的实例信息，存在新老实例并存的问题。如果说配置开放出去的话，可以根据在不同系统下SC的重启时间配置对应的TTL时间来解决或者降低对应问题出现的概率。

https://github.com/apache/servicecomb-service-center/blob/9ecab25a509057a3bfe8ab967b754562b6bff52b/server/core/microservice.go#L41C28-L41C28

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