Move snapshot history expire table properties to constants

I will be opening some PRs that will make it possible to implement the expire snapshot procedure. To make it easier to review, I'm breaking it down into smaller PRs.

@kevinjqliu, please take a look. I have tried many solutions, and this is the best I could come up with. The problem is that any import in this module triggers the deprecation message. The lesson I learned is to never have a constant at the module level. It’s better to namespace constants within classes, so we can deprecate them by simply creating a function as a property.

oh no, sorry for making this more difficult than it should be :( I think its okay to do this without the deprecation steps, but let's see what others think cc @sungwy @HonahX