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/rpc/codec/ssz.rs

@@ -56,9 +56,7 @@ impl<TSpec: EthSpec> Encoder<RPCCodedResponse<TSpec>> for SSZInboundCodec<TSpec>
                 RPCResponse::Pong(res) => res.data.as_ssz_bytes(),
                 RPCResponse::MetaData(res) => res.as_ssz_bytes(),
             },
-            RPCCodedResponse::InvalidRequest(err) => err.as_ssz_bytes(),
-            RPCCodedResponse::ServerError(err) => err.as_ssz_bytes(),
-            RPCCodedResponse::Unknown(err) => err.as_ssz_bytes(),
+            RPCCodedResponse::Error(_, err) => err.as_ssz_bytes(),
             RPCCodedResponse::StreamTermination(_) => {
                 unreachable!("Code error - attempting to encode a stream termination")
             }