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/availability-distribution/src/lib.rs

@@ -32,7 +32,7 @@ use sp_keystore::{CryptoStore, SyncCryptoStorePtr};
 
 use polkadot_erasure_coding::branch_hash;
 use polkadot_node_network_protocol::{
-	v1 as protocol_v1, NetworkBridgeEvent, PeerId, ReputationChange as Rep, View, OurView,
+	v1 as protocol_v1, PeerId, ReputationChange as Rep, View, OurView,
 };
 use polkadot_node_subsystem_util::metrics::{self, prometheus};
 use polkadot_primitives::v1::{
@@ -42,7 +42,7 @@ use polkadot_primitives::v1::{
 };
 use polkadot_subsystem::messages::{
 	AllMessages, AvailabilityDistributionMessage, AvailabilityStoreMessage, ChainApiMessage,
-	NetworkBridgeMessage, RuntimeApiMessage, RuntimeApiRequest,
+	NetworkBridgeMessage, RuntimeApiMessage, RuntimeApiRequest, NetworkBridgeEvent
 };
 use polkadot_subsystem::{
 	jaeger, errors::{ChainApiError, RuntimeApiError}, PerLeafSpan,
@@ -843,6 +843,15 @@ impl AvailabilityDistributionSubsystem {
 						);
 					}
 				}
+				FromOverseer::Communication {
+					msg: AvailabilityDistributionMessage::AvailabilityFetchingRequest(_),
+				} => { 
+					// TODO: Implement issue 2306:
+					tracing::warn!(
+						target: LOG_TARGET,
+						"To be implemented, see: https://github.com/paritytech/polkadot/issues/2306 !",
+					);
+				}
 				FromOverseer::Signal(OverseerSignal::ActiveLeaves(ActiveLeavesUpdate {
 					activated: _,
 					deactivated: _,