add an AddressVerified field to the ClientHelloInfo

[marten-seemann]

When implementing an IP-based blacklisting, it would be foolish to rely on unvalidated remote addresses: An attacker could easily prevent a legitimate client from connecting by just spoofing the source address.

The ClientHelloInfo should contain a field AddressVerified, and a blacklisting implementation would then only act on verified addresses. The number of unvalidated handshakes is limited by MaxUnvalidatesHandshakes, so this doesn’t pose an attack vector.