store: `HeightSub` should be able to notify callers of *any* new height written to the Store

Currently, heightSub keeps track of `height` which essentially the highest height written to the Store. Since Store now allows non-adjacent Appends, that `height` field is rendered useless (at least in the case of heightSub) as **it is not guaranteed that the highest header written to the store means that everything below is also written to the store**.

## Requested changes

* remove `height` field from `HeightSub` as it is no longer valuable

* `store.GetByHeight` should *first* do a `.Has` on the store to see if that height is already there, and then return the header if it exists in store -- and only if it does NOT yet exist, then subscribe to it via `heightSub`.

* `heightSub` should be able to "see" the height of every successful `Append` that happens on the store -- store should instead "Publish" to `heightSub` and `heightSub` notifs all of the callers of that height that it is now available.