Duplicate connections on resent SYN

We found a case where a lost ACK responding to an incoming SYN would result in another SYN sent from the client. In this case, the libutp stack would create a new connection context with the same peer address and connection ID and renotify the incoming_proc callback as if it were a new connection. With this change, the existing connection context is found and used for simply resending the ACK to the client and not notifying the incoming_proc callback.