MEDIUM: server/ssl: Base the SNI value to the HTTP host header by default

MEDIUM: server/ssl: Base the SNI value to the HTTP host header by default

For HTTPS outgoing connections, the SNI is now automatically set using the
Host header value if no other value is already set (via the "sni" server
keyword). It is now the default behavior. It could be disabled with the
"no-sni-auto" server keyword. And eventually "sni-auto" server keyword may
be used to reset any previous "no-sni-auto" setting. This option can be
inherited from "default-server" settings. Finally, if no connection name is
set via "pool-conn-name" setting, the selected value is used.

The automatic selection of the SNI is enabled by default for all outgoing
connections. But it is concretely used for HTTPS connections only. The
expression used is "req.hdr(host),host_only".

This patch should paritally fix the issue #3081. It only covers the server
part. Another patch will add the feature for HTTP health-checks.