MEDIUM: ssl: add sni support on the server lines

MEDIUM: ssl: add sni support on the server lines

The new "sni" server directive takes a sample fetch expression and
uses its return value as a hostname sent as the TLS SNI extension.
A typical use case consists in forwarding the front connection's SNI
value to the server in a bridged HTTPS forwarder :

   sni ssl_fc_sni