Currently Buffer header state is a 64-bit, but it is conceptually a bit field structure.
The CAS loops care about only some of those bits, however by operating
on a 64-bit structure unrelated field changes will cause a retry.
The atomic operations don't support a comparison mask, but in this case
we can at least CAS 32-bit, low and high half separately, reducing the likelihood
of retrial.
Regards,
Alexandre.
