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.

libnetwork/internal/kvstore/kvstore.go

@@ -77,7 +77,8 @@ type Store interface {
 	// DeleteTree deletes a range of keys under a given directory
 	DeleteTree(directory string) error
 
-	// Atomic operation on a single value
+	// Atomic CAS operation on a single value.
+	// Pass previous = nil to create a new key.
 	AtomicPut(key string, value []byte, previous *KVPair, options *WriteOptions) (bool, *KVPair, error)
 
 	// Atomic delete of a single value