serverState will be used by react on the first client render; this should avoid hydration mismatches when combined with the persist middleware, which can change the state between the SSR and the first CSR

this avoids hydration errors when state is restored from e.g. localstorage synchronously

this avoids hydration mismatches when updates happen to the store state between ssr and csr

if we default to `api.getState`, we will always read the client snapshot if there is no selector passed. An identity function returns its argument, which is either the snapshot (api.getState) or the server snapshot (api.getServerState)