Issue title: Scene.key conflicts with `PUSH` and `JUMP` with Tabbar
 Issue body: Hello everyone!

Im currently having some problem with the navigation used between `PUSH` and `JUMP`. The problem occurs in our app (that has a `tabbar` as a main component with other scenes as siblings). The problem occurs when the following sequence occurs: `Tabbar[Tab1] -> A1 -> B1 -> A2 -> B2 -> C -> D`
**IF** in `D` we execute `Action.Tab1()`, the navigation will perform a `JUMP` to `Tab1` in `Tabbar`. This all works fine until we want to `PUSH` `A3` to the stack. This will throw an error saying that `navigationState.children[2].key "scene_A" conflicts with another child`. Why does this happen?

The code is following:
```
    <Scene
        key="root"
        component={connect(store => ({
            session: store.Router.session,
            hasInitUser: store.Router.hasInitUser,
            }))(Switch)
        }
        tabs={true}
        unmountScenes
        selector={this._sceneSelector}
    >
        <Scene
            key="login"
            title='Login'
            component={Login}
        />
        <Scene
            key="main"
            title="main"
            >
                <Scene
                    key="tabBar"
                    tabs={true}
                    type={ActionConst.RESET}
                >
                    <Scene
                        key="tab1"
                        title='tab1'
                        hideNavBar={true}
                        component={Tab1}
                    />
                    <Scene
                        key="tab2"
                        title='tab2'
                        component={Tab2}
                    />
            </Scene>
        </Scene>
        <Scene
            key="A"
            title='A'
            component={A}
        />
        <Scene
            key="B"
            title='B'
            component={B}
        />
        <Scene
            key="C"
            title='C'
            component={C}
        />
        <Scene
            key="D"
            title='D'
            component={D}
        />
    </Scene>
```



 Comments: 
 Comment 0: 👍  I have the same problem. 



 Comment 1: @sirgalleto have you noticed any temporary solutions to this?
 Comment 2: Not yet, In my case I need to logout and show the Login page, maybe I will re-mount the router or anything like that. 
 Comment 3: 👍 
{'source': 'GithubIssues', 'repo_name_with_owner': 'aksonov/react-native-router-flux', 'issue_id': 203244327, 'repo_stars': 9750.0}