Fix undo & edit last operation & redo handling

Summary

Undo is quite unstable during a multi-user session

• Addon version: all
• Blender version: all
• OS: all

Steps to reproduce

Hit Ctrl+Z multiple time during a session.

What is the current bug behavior?

On each undo, the multi-user run a sanity check on each node of the replication graph.

This test consist of checking if each tacked datablock are still existing.

Possible fixes

In order to increase the undo/redo stability, we need to:

• Lock background threads during the sanity check