Changed behavior of scenes into stack (always opens the first one after switching)

### Version

- react-native-router-flux v4.0.1
- react v16.5
- react-native v0.57

### Expected behaviour

I'd like to save behaviour from previous versions of scenes into stacks.

We have stacks with scenes (tab stacks, actually). We navigate in one stack between scenes, get scene №3 (for example) and tap on tab for entering another stack.

When we'll open previous stack, we get scene №3 (like at the moment we left this stack) and continue to use the app

### Actual behaviour

When we'll open previous stack, we'll lose our state and see a pop to first scene in stack.