Change the default value of the bthread_tag in class ServerOptions to BTHREAD_TAG_DEFAULT

[MJY-HUST]

What problem does this PR solve?

将ServerOptions 中的bthread_tag默认值改为BTHREAD_TAG_DEFAULT && 简化代码逻辑 Issue Number:

Problem Summary:

What is changed and the side effects?

Changed:

Side effects:

• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
• Please follow Contributor Covenant Code of Conduct.(请遵循贡献者准则).

[wwbmmm]

LGTM

[yanglimingcn]

LGTM 👍