Generic request/response infrastructure for Polkadot (#2352)

* Move NetworkBridgeEvent to subsystem::messages.

It is not protocol related at all, it is in fact only part of the
subsystem communication as it gets wrapped into messages of each
subsystem.

* Request/response infrastructure is taking shape.

WIP: Does not compile.

* Multiplexer variant not supported by Rusts type system.

* request_response::request type checks.

* Cleanup.

* Minor fixes for request_response.

* Implement request sending + move multiplexer.

Request multiplexer is moved to bridge, as there the implementation is
more straight forward as we can specialize on `AllMessages` for the
multiplexing target.

Sending of requests is mostly complete, apart from a few `From`
instances. Receiving is also almost done, initializtion needs to be
fixed and the multiplexer needs to be invoked.

* Remove obsolete multiplexer.

* Initialize bridge with multiplexer.

* Finish generic request sending/receiving.

Subsystems are now able to receive and send requests and responses via
the overseer.

* Doc update.

* Fixes.

* Link issue for not yet implemented code.

* Fixes suggested by @ordian - thanks!

- start encoding at 0
- don't crash on zero protocols
- don't panic on not yet implemented request handling

* Update node/network/protocol/src/request_response/v1.rs

Use index 0 instead of 1.

Co-authored-by: Andronik Ordian <write@reusable.software>

* Update node/network/protocol/src/request_response.rs

Co-authored-by: Andronik Ordian <write@reusable.software>

* Fix existing tests.

* Better avoidance of division by zoro errors.

* Doc fixes.

* send_request -> start_request.

* Fix missing renamings.

* Update substrate.

* Pass TryConnect instead of true.

* Actually import `IfDisconnected`.

* Fix wrong import.

* Update node/network/bridge/src/lib.rs

typo

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

* Update node/network/bridge/src/multiplexer.rs

Remove redundant import.

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

* Stop doing tracing from within `From` instance.

Thanks for the catch @tomaka!

* Get rid of redundant import.

* Formatting cleanup.

* Fix tests.

* Add link to issue.

* Clarify comments some more.

* Fix tests.

* Formatting fix.

* tabs

* Fix link

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

* Use map_err.

Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

* Improvements inspired by suggestions by @drahnr.

- Channel size is now determined by function.
- Explicitely scope NetworkService::start_request.

Co-authored-by: Andronik Ordian <write@reusable.software>
Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>
Co-authored-by: Bernhard Schuster <bernhard@ahoi.io>

polkadot/node/network/protocol/src/request_response/v1.rs

@@ -0,0 +1,44 @@
+// Copyright 2021 Parity Technologies (UK) Ltd.
+// This file is part of Polkadot.
+
+// Polkadot 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.
+
+// Polkadot 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 Polkadot.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Requests and responses as sent over the wire for the individual protocols.
+
+use parity_scale_codec::{Decode, Encode};
+
+use polkadot_primitives::v1::{CandidateHash, ErasureChunk, ValidatorIndex};
+
+use super::request::IsRequest;
+use super::Protocol;
+
+/// Request an availability chunk.
+#[derive(Debug, Clone, Encode, Decode)]
+pub struct AvailabilityFetchingRequest {
+	candidate_hash: CandidateHash,
+	index: ValidatorIndex,
+}
+
+/// Receive a rqeuested erasure chunk.
+#[derive(Debug, Clone, Encode, Decode)]
+pub enum AvailabilityFetchingResponse {
+	/// The requested chunk.
+	#[codec(index = 0)]
+	Chunk(ErasureChunk),
+}
+
+impl IsRequest for AvailabilityFetchingRequest {
+	type Response = AvailabilityFetchingResponse;
+	const PROTOCOL: Protocol = Protocol::AvailabilityFetching;
+}