Update libkv dependency

docs/configuration/backends/etcd.md

@@ -31,6 +31,16 @@ watch = true
 #
 prefix = "/traefik"
 
+# Force to use API V3 (otherwise still use API V2)
+#
+# Deprecated
+#
+# Optional
+# Default: false
+#
+useAPIV3 = true
+
+
 # Override default configuration template.
 # For advanced users :)
 #
@@ -59,3 +69,7 @@ prefix = "/traefik"
 To enable constraints see [backend-specific constraints section](/configuration/commons/#backend-specific).
 
 Please refer to the [Key Value storage structure](/user-guide/kv-config/#key-value-storage-structure) section to get documentation on Traefik KV structure.
+
+!!! note
+    The option `useAPIV3` allows using Etcd API V3 only if it's set to true.
+    This option is **deprecated** and API V2 won't be supported in the future.

docs/user-guide/kv-config.md

@@ -322,6 +322,10 @@ As a result, it may be possible for Træfik to read an intermediate configuratio
 To solve this problem, Træfik supports a special key called `/traefik/alias`.
 If set, Træfik use the value as an alternative key prefix.
 
+!!! note
+    The field `useAPIV3` allows using Etcd V3 API which should support updating multiple keys atomically with Etcd.
+    Etcd API V2 is deprecated and, in the future, Træfik will support API V3 by default.
+
 Given the key structure below, Træfik will use the `http://172.17.0.2:80` as its only backend (frontend keys have been omitted for brevity).
 
 | Key                                                                     | Value                       |