Enable splice for freedom outbound (downlink only)

- Add outbound name
- Add outbound conn in ctx
- Refactor splice: it can be turn on from all inbounds and outbounds
- Refactor splice: Add splice copy to vless inbound
- Fix http error test
- Add freedom splice toggle via env var
- Populate outbound obj in context
- Use CanSpliceCopy to mark a connection
- Turn off splice by default

proxy/shadowsocks/server.go

@@ -71,6 +71,10 @@ func (s *Server) Network() []net.Network {
 }
 
 func (s *Server) Process(ctx context.Context, network net.Network, conn stat.Connection, dispatcher routing.Dispatcher) error {
+	inbound := session.InboundFromContext(ctx)
+	inbound.Name = "shadowsocks"
+	inbound.SetCanSpliceCopy(3)
+	
 	switch network {
 	case net.Network_TCP:
 		return s.handleConnection(ctx, conn, dispatcher)
@@ -110,13 +114,7 @@ func (s *Server) handleUDPPayload(ctx context.Context, conn stat.Connection, dis
 	})
 
 	inbound := session.InboundFromContext(ctx)
-	if inbound == nil {
-		panic("no inbound metadata")
-	}
-	inbound.Name = "shadowsocks"
-
 	var dest *net.Destination
-
 	reader := buf.NewPacketReader(conn)
 	for {
 		mpayload, err := reader.ReadMultiBuffer()