Add state storage

[ro31337]

StateStorage keeps state data, updates state when underlying storage updates. Objects in application can be stateful. StateStorage is basic structure that implements persistent state (won't go away on app restart).

Key for constructor below is already existing new StateKey(...).toString() - something that identifies particular command for particular user for particular platform.

StateStorage implementation:

constructor(key) - Constructor. key - state key, for example telegram_31337_4566bd48-d369-4594-aa1e-fb5dae1acf43.
load() - Loads state from the actual storage. Returns promise which is executed when changes were initially loaded.
save() - updates the storage with current state, returns promise. Will only overwrite the children enumerated in state.
setState({ ... }) - updates selected properties.
state - actual state.
dispose - unsubscribes itself from storage updates (off method for Firebase).

Usage example:

new StateStorage('telegram_31337_4566bd48-d369-4594-aa1e-fb5dae1acf43')
    .load()
    .then((storage) => {
        console.dir(storage.state); // => {}
        storage.setState({ a: 1, b: 2 });
        storage.save().then(() => {
            console.log('saved');
        })
    });