Allow individual reducers to track history specific state

[webmasterkai]

createHistoryCache and makeHydratable(reducer) are hacks to make a proof of concept work. It should be up to the individual reducer to decide if it needs to reset itself on history changes. Perhaps a higher order reducer function that accepts reducers and gives them state based current the current history key.

state = {
  history: {},
  historySession: {
    xwu39s: {
      filter: {
        // Some filter state specific to the xwu39s history.
      }
    }
  }
}

There would also need to be some selector functions to make it easy to select the state for current history session but that's easy enough. I like this direction much more...