Refactor inbound substream logic with async (#1325)

## Issue Addressed
#1112 

The logic is slightly different but still valid wrt to error handling.
- Inbound state is either Busy with a future that return the subtream (and info about the processing)
- The state machine works as follows:
  - `Idle` with pending responses => `Busy`
  - `Busy` => finished ? if so and there are new pending responses then `Busy`, if not then `Idle`
               => not finished remains `Busy`
- Add an `InboundInfo` for readability
- Other stuff:
  - Close inbound substreams when all expected responses are sent
  - Remove the error variants from `RPCCodedResponse` and use the codes instead
  - Fix various spelling mistakes because I got sloppy last time

Sorry for the delay

Co-authored-by: Age Manning <Age@AgeManning.com>

beacon_node/eth2_libp2p/src/behaviour/mod.rs

@@ -256,11 +256,8 @@ impl<TSpec: EthSpec> Behaviour<TSpec> {
         error: RPCResponseErrorCode,
         reason: String,
     ) {
-        self.eth2_rpc.send_response(
-            peer_id,
-            id,
-            RPCCodedResponse::from_error_code(error, reason),
-        )
+        self.eth2_rpc
+            .send_response(peer_id, id, RPCCodedResponse::Error(error, reason.into()))
     }
 
     /* Peer management functions */