Update libkv dependency

vendor/github.com/docker/libkv/store/store.go

@@ -12,12 +12,16 @@ type Backend string
 const (
 	// CONSUL backend
 	CONSUL Backend = "consul"
-	// ETCD backend
+	// ETCD backend with v2 client (backward compatibility)
 	ETCD Backend = "etcd"
+	// ETCDV3 backend with v3 client
+	ETCDV3 Backend = "etcdv3"
 	// ZK backend
 	ZK Backend = "zk"
 	// BOLTDB backend
 	BOLTDB Backend = "boltdb"
+	// REDIS backend
+	REDIS Backend = "redis"
 )
 
 var (
@@ -44,6 +48,7 @@ type Config struct {
 	ClientTLS         *ClientTLSConfig
 	TLS               *tls.Config
 	ConnectionTimeout time.Duration
+	SyncPeriod        time.Duration
 	Bucket            string
 	PersistConnection bool
 	Username          string
@@ -67,20 +72,20 @@ type Store interface {
 	Put(key string, value []byte, options *WriteOptions) error
 
 	// Get a value given its key
-	Get(key string) (*KVPair, error)
+	Get(key string, options *ReadOptions) (*KVPair, error)
 
 	// Delete the value at the specified key
 	Delete(key string) error
 
 	// Verify if a Key exists in the store
-	Exists(key string) (bool, error)
+	Exists(key string, options *ReadOptions) (bool, error)
 
 	// Watch for changes on a key
-	Watch(key string, stopCh <-chan struct{}) (<-chan *KVPair, error)
+	Watch(key string, stopCh <-chan struct{}, options *ReadOptions) (<-chan *KVPair, error)
 
 	// WatchTree watches for changes on child nodes under
 	// a given directory
-	WatchTree(directory string, stopCh <-chan struct{}) (<-chan []*KVPair, error)
+	WatchTree(directory string, stopCh <-chan struct{}, options *ReadOptions) (<-chan []*KVPair, error)
 
 	// NewLock creates a lock for a given key.
 	// The returned Locker is not held and must be acquired
@@ -88,7 +93,7 @@ type Store interface {
 	NewLock(key string, options *LockOptions) (Locker, error)
 
 	// List the content of a given prefix
-	List(directory string) ([]*KVPair, error)
+	List(directory string, options *ReadOptions) ([]*KVPair, error)
 
 	// DeleteTree deletes a range of keys under a given directory
 	DeleteTree(directory string) error
@@ -117,6 +122,16 @@ type WriteOptions struct {
 	TTL   time.Duration
 }
 
+// ReadOptions contains optional request parameters
+type ReadOptions struct {
+	// Consistent defines if the behavior of a Get operation is
+	// linearizable or not. Linearizability allows us to 'see'
+	// objects based on a real-time total order as opposed to
+	// an arbitrary order or with stale values ('inconsistent'
+	// scenario).
+	Consistent bool
+}
+
 // LockOptions contains optional request parameters
 type LockOptions struct {
 	Value     []byte        // Optional, value to associate with the lock