Issue 517: Issue when doing StartHost after a StopHost call.

Repository: miragenet/mirage

Issue body:
**Describe the bug**
When stopping the server and then starting it again causes all scene objects with network behaviours to get destroyed.

And when running the project in the Unity Editor.
All scene objects with network behaviours get destroyed on the first StartHost call instead. To fix this you have either restarted unity or force a script compile.

**Repro project**
https://drive.google.com/file/d/1T-wNEG8RdG-I30N9prCUU2WQdF707kvm/view?usp=sharing

**To Reproduce**
Steps to reproduce the behavior:
1. Open the attached project
2. Open Scenes/SampleScene
3. Enter play mode
4. Look at the NetworkBehaviour GameObject being destroyed on StartHost the second time.

**Expected behavior**
Scene Objects with **Network Behaviours** should not be deleted when doing **StopHost**.

**Desktop:**
* OS: \[Windows]
* Unity version: \[2020.2.1f1]
* Mirror branch: \[master]
