Issue 467: Networked objects not being destroyed after StopHost in host mode

Repository: miragenet/mirage

Issue body:
**Describe the bug**
After stopping the host mode via `StopHost`, the objects spawned by the server in host scene are not properly destroyed. Their "Is Server" and "Is Client" properties are both set to false though.

**Repro project**
Attached a simple repro project.

**To Reproduce**
Steps to reproduce the behavior:
1. Open the attached project '...'
2. Start the game
3. Spawn some networked objects by the player by clicking left mouse button
4. Click Stop Host in top left corner
5. Observe the objects not being destroyed in the scene.

**Expected behavior**
There should be no networked objects left after the server is stopped.

**Desktop:**
* OS: Windows 10 64bit
* Build target: Windows standalone
* Unity version: 2020.1.11
* Mirror branch: latest master
[MirrorNGReproNoDestroy.zip](https://github.com/MirrorNG/MirrorNG/files/5482263/MirrorNGReproNoDestroy.zip)
