Open one substream for each notifications protocol (#4909)

* Open one substream for each notifications protocol

* Fix WASM build

* Apply suggestions from code review

Co-Authored-By: Toralf Wittner <tw@dtex.org>

* Address concerns

* Use unsigned-varint to read the varint

* Use unsigned-varint

* Forgot Cargo.lock

Co-authored-by: Toralf Wittner <tw@dtex.org>

substrate/client/network/src/protocol/generic_proto/handler.rs

@@ -0,0 +1,22 @@
+// Copyright 2020 Parity Technologies (UK) Ltd.
+// This file is part of Substrate.
+
+// Substrate is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Substrate is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
+
+pub use self::group::{NotifsHandlerProto, NotifsHandler, NotifsHandlerIn, NotifsHandlerOut};
+
+mod group;
+mod legacy;
+mod notif_in;
+mod notif_out;