Add stress test for atomic_shared_ptr
Add stress test for atomic_shared_ptr

Summary: None of the existing unit tests exercises the local -> global refcount transfer, which is the most critical part in `atomic_shared_ptr` in terms of correctness, so add a stress test that `load()`s the pointer enough times to trigger a transfer.

Reviewed By: dmm-fb

Differential Revision: D39927439

fbshipit-source-id: a8a50e33dd4afd1bd7bf0a2c309d92503bfb949c

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