BUG/MEDIUM: server: Use sni as pool connection name for SSL server only

BUG/MEDIUM: server: Use sni as pool connection name for SSL server only

By default, for a given server, when no pool-conn-name is specified, the
configured sni is used. However, this must only be done when SSL is in-use
for the server. Of course, it is uncommon to have a sni expression for
now-ssl server. But this may happen.

In addition, the SSL may be disabled via the CLI. In that case, the
pool-conn-name must be discarded if it was copied from the sni. And, we must
of course take care to set it if the ssl is enabled.

Finally, when the attac-srv action is checked, we now checked the
pool-conn-name expression.

This patch should be backported as far as 3.0. It relies on "MINOR: server:
Parse sni and pool-conn-name expressions in a dedicated function" which
should be backported too.