Russh: Unbounded memory exhaustion via CHANNEL_OPEN flood during a client-stalled rekey

Summary A russh server can be driven to unbounded heap growth process OOM / kill by a peer that speaks only standard SSH messages, in the default configuration. The peer starts a key re-exchange sends SSHMSGKEXINIT but never sends the follow-up SSHMSGKEXECDHINIT, leaving the server's kex state...