Small FlatMap optimization with default initialization

[chenBright]

What problem does this PR solve?

Issue Number:

Problem Summary:

目前FlatMap是两阶段初始化：构造函数+init函数。这种初始化方式很容易用错，甚至出core，所以使用上必须小心翼翼，体验不好。

What is changed and the side effects?

Changed:

FlatMap构造完成即完成初始化。使用sso、IOBuf::SmallView的策略，初始化_buckets和_thumbnail使用类内的内存块，没有init函数中Alloc失败的顾虑。

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).

[javeme]

Because the usage difference with std::map, FlatMap is unfriendly to use.
I think this PR is a good improvement.

[chenBright]

There are some unknown crash issues. This PR will be opened after these issues are resolved.