pass current state to subscribers

[matanwerbner]

at least in it's vanilla form - it will be usefull if store.subscribe() will have the current state as an argument, instead of needlessly calling store.getState() inside.

[markerikson]

This has been proposed numerous times, and is not viable, primarily due to interference with batched dispatches. Please see #580 and #2214 for discussion.

[gaearon]

I don't think it has to do with batching. Batching is problematic when we pass action. However passing state would be fine. The problem with it is that it would burden any enhancers (e.g. DevTools) with reimplementing subscriptions. This is solved by #1702 but we didn't go ahead with it, and I'm not confident about direction in #2214, so we likely won't ever fix this.