Clear pending session data if transport handshake fails

[str4d]

Currently, if a message is sent to an unconnected peer, but the transport handshake fails, the message persists in the pending session cache for that transport. When a subsequent message is sent to that peer, the presence of the peer in the pending session cache means that it is also cached, on the assumption that a session is pending. The result is that if a connection fails to a peer due to transient issues, it can never be reached again within the current router.