BUG/MEDIUM: server/ssl: Unset the SNI for new server connections if none is set

BUG/MEDIUM: server/ssl: Unset the SNI for new server connections if none is set

When a new SSL server connection is created, if no SNI is set, it is
possible to inherit from the one of the reused TLS session. The bug was
introduced by the commit 95ac5fe4a ("MEDIUM: ssl_sock: always use the SSL's
server name, not the one from the tid"). The mixup is possible between
regular connections but also with health-checks connections.

To fix the issue, when no SNI is set, for regular server connections and for
health-check connections, the SNI must explicitly be disabled by calling
ssl_sock_set_servername() with the hostname set to NULL.

Many thanks to Lukas for his detailed bug report.

This patch should fix the issue #3195. It must be backported as far as 3.0.