Bump RPC version after ReadFile RPC arguments change (#6457)

ReadFile RPC input argument has been changed in commit a8f5939452959d27674560c6b803daa9,
however, RPC doesn't detect such a change when it calls other nodes with older versions.

Hence, bumping RPC version.

Fixes #6458

cmd/globals.go

@@ -206,8 +206,9 @@ var (
 	// RPC V1 - Initial version
 	// RPC V2 - format.json XL version changed to 2
 	// RPC V3 - format.json XL version changed to 3
+	// RPC V4 - ReadFile() arguments signature changed
 	// Current RPC version
-	globalRPCAPIVersion = RPCVersion{3, 0, 0}
+	globalRPCAPIVersion = RPCVersion{4, 0, 0}
 
 	// Allocated etcd endpoint for config and bucket DNS.
 	globalEtcdClient *etcd.Client