do not disconnect on receiving canceled request (#3869)

LC cancels concurrent requests if one peer sent a correct response and
waiting for other peers is no longer useful. On the server side this
resulted in a descore (-500) and a likely disconnect. The behaviour is
changed to match `UnexpectedEOF`, `PotentiallyExpectedEOF` handling that
return an error response without disconnecting from the peer.

beacon_chain/networking/eth2_network.nim

@@ -1024,8 +1024,12 @@ proc handleIncomingStream(network: Eth2Node,
       try:
         awaitWithTimeout(
           readChunkPayload(conn, peer, maxChunkSize(MsgRec), MsgRec), deadline):
-            returnInvalidRequest(
+            # Timeout, e.g., cancellation due to fulfillment by different peer.
+            # Treat this similarly to `UnexpectedEOF`, `PotentiallyExpectedEOF`.
+            await sendErrorResponse(
+              peer, conn, InvalidRequest,
               errorMsgLit "Request full data not sent in time")
+            return
 
       except SerializationError as err:
         returnInvalidRequest err.formatMsg("msg")