Eliminate IO/logging initiated by unauthenticated code paths

[stv0g]

By @koraa in #27 (comment)

https://github.com/stv0g/go-rosenpass/blob/d7e38ecaf9e7803f2824a03ac24ac34944a53af6/handshake_initiator.go#L101

https://github.com/stv0g/go-rosenpass/blob/d7e38ecaf9e7803f2824a03ac24ac34944a53af6/server.go#L143

Severity: 3 (Dangerous) – No IO should be performed in response to maliciously crafted messages as performing IO opens up pandoras box in regards to DOS attacks (think of logs filling up, disk exhaustion, ghastly stuff like that).

Note that this performs a lot of allocations even if log level Debug was used.