Entities extraction is main thread heavy

Steps to reproduce:

1. See https://discussions.unity.com/t/surface-cache-gi-preview/1720494/405

Actual results: Entity extraction runs on the main thread

Expected results: Entity extraction goes wide

Reproducible with versions: 7000.0x, 6000.7x

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): 

Notes:

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