[RED-32] Remove array form for configureStore.middleware argument

[markerikson]

We've seen too many cases where users call configureStore({reducer, middleware: [myMiddleware]}), and accidentally blow away the defaults.

Let's remove that form and only allow callbacks. We already did that for enhancers, so same idea.

Users can still do middleware: () => [myMiddleware] if they really want to, but that at least forces them to make use of the callback form, and reduces the chance of it being a mistake.

_RED-32