postgres-protocol: Unbounded SCRAM iteration count allows a malicious server to cause CPU-exhaustion denial of service

A malicious, compromised, or man-in-the-middle server can supply an arbitrarily large SCRAM-SHA-256 PBKDF2 iteration count during authentication. The client runs it inline with no upper bound, pinning a tokio worker thread for minutes per connection, possibly stalling the whole async runtime...