Based on the paper: "OpenVPN is Open to VPN Fingerprinting"
See: https://www.usenix.org/conference/usenixsecurity22/presentation/xue-diwen
Basic idea:
* the distribution of the first byte of the messages (i.e. the distribution
of the op-codes) is quite unique
* this fingerprint might be still detectable even if the OpenVPN packets are
somehow fully encrypted/obfuscated
The heuristic is disabled by default.
