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/drain.go

@@ -0,0 +1,10 @@
+package drain
+
+import "io"
+
+//go:generate go run github.com/xtls/xray-core/common/errors/errorgen
+
+type Drainer interface {
+	AcknowledgeReceive(size int)
+	Drain(reader io.Reader) error
+}