issue of handle registry server online message

## Why you need it?
Is your feature request related to a problem? Please describe in details

当前，收到注册中心服务器上线消息时，并没有进行实时处理
currently, The message that the server goes online is not processed when receive server online message from registry center.

## How it could be?
A clear and concise description of what you want to happen. You can explain more about input of the feature, and output of it.

## Other related information
Add any other context or screenshots about the feature request here.

Start by tracing how Seata receives and dispatches server-online messages from the registry center, then identify the existing server event handling path. Define the expected real-time behavior with maintainers and add coverage showing that an online message is processed promptly.

Written by the indexing model from the issue text.