Make apps handle inflight handshake request #331
Description
During a persistent connection, the client side of an entity can send another handshake frame to refresh the session keys and start with a fresh connection state without disconnecting.
However, in our current application flow, we do not support this behavior. We assume that the handshake occurs only at the very beginning of the connection and will never happen again later to refresh session keys or perform rekeying. Because of this assumption, if another client attempts such a handshake to refresh the state, we might interpret it as an attack and disconnect the client.