Fix restored promise for multiple plugin instances

[PeterBurner]

• see store.restored Promise resolves before the state is restored. #161

[PeterBurner]

it would be nice if one could assign a custom name to each plugin so its possible to only wait for a subset
mayby somthing like this:

(store as any).restoredAll = (store as any).restoredAll || {};
(store as any).restoredAll[this.pluginName] = (this.restoreState(this.key, this.storage) as Promise<S>).then(savedState => {

...

(store as any).restored = Promise.all(Object.values((store as any).restoredAll));