MINOR: acl: add support for TLS server name matching using SNI

MINOR: acl: add support for TLS server name matching using SNI

Server Name Indication (SNI) is a TLS extension which makes a client
present the name of the server it is connecting to in the client hello.
It allows a transparent proxy to take a decision based on the beginning
of an SSL/TLS stream without deciphering it.

The new ACL "req_ssl_sni" matches the name extracted from the TLS
handshake against a list of names which may be loaded from a file if
needed.