Redux SDK

What did you accomplish?

Updated initSplitSdk action creator to attach event listeners only once (not when the thunk action is dispatched). This is needed for server-side, where initSplitSdk is called once but the created thunk is dispatched each time a new store is created per request/session.
Updated destroySplitSdk action creator to shut down the SDK before the thunk action is dispatched. This is needed for server-side, where destroySplitSdk is invoked without dispatching the thunk action.
Added onDestroy callback parameter to destroySplitSdk, to listen when the SDK has gracefully shut down. This param is mainly needed for server-side, where the thunk action is not dispatched and so the user cannot access the destroy promise as in client-side: store.dispatch(destroySplitSdk()).then(...).

E2E test to evaluate that event callbacks are invoked only once when initSplitSdk thunk is dispatched multiple times.
E2E test to evaluate that onDestroy callback is invoked even when the returned destroySplitSdk thunk action is not dispatched.

[DONE: onDestroy invoked preferably after action is dispatched] define if onDestroy callback must be invoked after splitDestroy action is dispatched or not.