Add fetched field to async data structure.

[Bloomca]

Currently for async data I use the next structure:

{
  isPending: true,
  data: null,
  error: null,
};

I think that adding fetched will be beneficial for couple of reasons:

• we can legitimately get null as a result, so data field can contain it after performing action
• it is much easier to use it inside shouldBeFetched. Also, if the user uses our middleware (or provides promises storage to thunk), we can safely delegate isPending check to the promisesStorage.

[Bloomca]

Done by #15