Rename weakCompareAndSetVolatile to weakCompareAndSet

Rename weakCompareAndSet to weakCompareAndSetPlain and weakCompareAndSetVolatile to weakCompareAndSet, thereby leveraging a consistent naming scheme for read-modify-write methods.

The Atomic* classes should be updated in a separate issue to:

1) deprecate (not for removal) weakCompareAndSet
2) add weakCompareAndSetPlain, and leave weakCompareAndSetVolatile (which remains the sole inconsistency in the naming scheme)