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Add an session-wise uuid integer to IDs.

"session-wise" here mean while editing a same .blend file. So creating
or opening a new one will reset the uuid counter. This should avoid any
overflow in practice.

Only IDs added to Main database get an uuid, runtime-only ones are not
affected.

This is intended to provide undo with a way to find IDs across several
'memory realms' (undo speedup project).

No behavior change is expected from this commit itself.

Part of T60695.

Differential Revision: https://developer.blender.org/D7007