Improve naming conventions in Entity implementation

As discussed here: https://github.com/Azure/azure-functions-durable-python/pull/184

Many parts of our Entity implementation have overloaded terms. For instance, "entity state" is a class name and a variable. This is also true for our JS implementation. We should go over all Entity implementation components and improve their organization and naming.

首先阅读 Entity 的实现以及 pull request #184 中的讨论。盘点含义重叠的术语，以及需要更清晰组织和命名的 Entity 组件，包括 issue 中提到的 JavaScript 实现。完成的标准是 Entity 的各个实现始终使用一致且明确无歧义的名称。

由索引模型根据 Issue 内容生成。