Unified drain support for vmess and shadowsocks (#791)

* Added test for no terminate signal

* unified drain support for vmess and shadowsockets

* drain: add generated file

Co-authored-by: Shelikhoo <xiaokangwang@outlook.com>

common/drain/errors.generated.go

@@ -0,0 +1,9 @@
+package drain
+
+import "github.com/xtls/xray-core/common/errors"
+
+type errPathObjHolder struct{}
+
+func newError(values ...interface{}) *errors.Error {
+	return errors.New(values...).WithPathObj(errPathObjHolder{})
+}