After #3655 fix, atomic wait on shared_ptr has timed backoff for the case of control blocks difference but value match.

We need to consider alternatives to this timed backoff for vNext.

WaitOnAddress on 128 bits may be invented by then.

Otherwise, may consider making atomic<shared_ptr> use a different approach, like the fallback we have for non-lock-free atomics.

vNext note: Resolving this issue will require breaking binary compatibility. We won't be able to accept pull requests for this issue until the vNext branch is available. See #169 for more information.