Refactor: move slave pkg inside of cluster

Test: middleware for node communication

pkg/cluster/errors.go

@@ -1,8 +1,12 @@
 package cluster
 
-import "errors"
+import (
+	"errors"
+	"github.com/cloudreve/Cloudreve/v3/pkg/serializer"
+)
 
 var (
 	ErrFeatureNotExist = errors.New("No nodes in nodepool match the feature specificed")
 	ErrIlegalPath      = errors.New("path out of boundary of setting temp folder")
+	ErrMasterNotFound  = serializer.NewError(serializer.CodeMasterNotFound, "未知的主机节点", nil)
 )