Revert "Use `sync.Mutex` rather than a spin lock for the threading example"
Revert "Use `sync.Mutex` rather than a spin lock for the threading example"

This reverts commit 8d1f46d837efdcebaf2e3660942a93337a230329.

gingerBill/Odin,gingerBill/Odin,gingerBill/Odin,gingerBill/Odin