Improve `x/posts` structures names

[leobragaz]

After reviewing the #893 I've realised that there are some confusing structures names that don't really express the intend of those types. I think both SubspaceDataEntry and PostDataEntry should be renamed to a more meaningful SubspaceRelatedIDs and PostRelatedIDs. This will help to give a better context to the the types and ease the life of devs understanding what they're about.

[RiccardoM]

Those are internal structures, used only when importing/exporting the genesis state. They shouldn't be used by external developers and hence their names. I don't think it's necessary to change them

[leobragaz]

Also for our internal usage that would be useful.

[RiccardoM]

Let's make this extremely low priority then