API Change: AtomicPut with previous = nil is create.
AtomicPut can now be used to Compare-and-Swap against the state
where the key doesn't yet exist. E.g. a race where two clients
create the same key: one succeeds, the other fails.
Pass nil for the previous argument of AtomicPut for this
behavior. Before, this would cause an error.
Implements this change for all three backends.