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/protocol.rs

@@ -323,12 +323,12 @@ impl<TSpec: EthSpec> RPCRequest<TSpec> {
     /* These functions are used in the handler for stream management */
 
     /// Number of responses expected for this request.
-    pub fn expected_responses(&self) -> usize {
+    pub fn expected_responses(&self) -> u64 {
         match self {
             RPCRequest::Status(_) => 1,
             RPCRequest::Goodbye(_) => 0,
-            RPCRequest::BlocksByRange(req) => req.count as usize,
-            RPCRequest::BlocksByRoot(req) => req.block_roots.len(),
+            RPCRequest::BlocksByRange(req) => req.count,
+            RPCRequest::BlocksByRoot(req) => req.block_roots.len() as u64,
             RPCRequest::Ping(_) => 1,
             RPCRequest::MetaData(_) => 1,
         }