Stores 'writeThis' to the memory referenced by 'here' if the value
referenced by 'here' is equal to the value referenced by 'ifThis'.
The prior value referenced by 'here' is written to ifThis and
returned to the user.
The 'weak' version of cas may spuriously fail. It is recommended to
use casWeak only when cas would be used in a loop.
This operation is both lock-free and atomic.
Stores 'writeThis' to the memory referenced by 'here' if the value referenced by 'here' is equal to the value referenced by 'ifThis'. The prior value referenced by 'here' is written to ifThis and returned to the user. The 'weak' version of cas may spuriously fail. It is recommended to use casWeak only when cas would be used in a loop. This operation is both lock-free and atomic.