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.

Start by reading the Entity implementation and the discussion in pull request #184. Inventory the overloaded terms and the Entity components that need clearer organization and naming, including the JavaScript implementation mentioned in the issue. Done means the Entity implementations use consistent, unambiguous names throughout.

索引モデルが issue の本文から書いたものです。