fix: Prevent StopClient from running twice (MirrorNetworking#2953)

* fix: Prevent StopClient from running twice
- Fixes MirrorNetworking#2940

* Added comments

* Fixed stupid mistake

* moved check to OnClientDisconnect
Still need to fire that virtual so it can be overridden.

* Added check to StopClient itself too

breaking: SyncObject as class instead of interface (MirrorNetworking#…

…2947)

* breaking: SyncObject as class instead of interface

* classes inheriting from SyncObject don't all need to implement obsoleted Flush() anymore

* IsRecording default moved to base class

* OnDirty default moved to base class

* comment

* IsRecording simplified now that it's a class

* OnDirty simplified now that it's a class

fix: MirrorNetworking#2912 ILPostProcessor isn't used before 2020.3 b…

…ecause Unity bugs

fix: SyncVarGameObject/NetworkIdentityEqual not being accessible by w…

…eaved NetworkBehaviour components from outside assemblies

fix: only IEnumerable SyncObjects are drawn as SyncObject Collections…

… in Inspector now.

previously SyncVar<T> SyncObject would've been drawn as a list too.

Make `NetworkConnection.observing` public (MirrorNetworking#2942)

- NetworkIdentity.observers is already public

Weaver: SyncVarProcessor renamed to SyncVarAttributeProcessor

NetworkBehaviour.HasSyncObjects: internal is enough

explain SyncVar hook guard

feat: Implemented ChangeOwnerMessage (MirrorNetworking#2924)

* Implemented ChangeOwnerMessage
- Separates ownership change from Spawn message
- NetworkIdentityTests updated

* commented log