reconstruction the getty module

[georgehao]

What would you like to be added:

Why is this needed:

[iSuperCoder]

@georgehao
你是基于什么原因想要重构getty模块的？我这里也发现了getty模块的一点问题，可能需要重构来解决。

问题如下：
getty模块所依赖的getty.Session接口中，定义了小写开头的私有方法。这导致我们无法在seata-go中对Session接口进行mock，不方便单元测试，影响测试覆盖率。

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@georgehao
What is your reason for wanting to refactor the getty module? I also found a little problem with the getty module here, which may need to be refactored to solve it.

Questions are as follows:
In the getty.Session interface on which the getty module depends, private methods starting with lowercase are defined. This makes it impossible for us to mock the Session interface in seata-go, which is inconvenient for unit testing and affects test coverage.

[georgehao]

getty 到处都是用的单例模式，这点不太能接受。模块跟模块之间的联系表达的不清晰

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The singleton pattern used everywhere in getty is not acceptable. The relationship between modules and modules is not clearly expressed