SSU2: split() and send in-session termination after certain session confirmed errors

containing a specific termination reason,
and create a post-termination handler directly from IES2 for any additional packets.
Don't do this unless we get intro key and validate static key in RI.
Don't blocklist the IP for a banned router unless we have validated
the static key to prevent spoofing.
Don't queue data packet after failure
Don't send additional terminations after receiving termination ack