BUG/MEDIUM: ssl: Don't reuse TLS session if the connection's SNI differs

BUG/MEDIUM: ssl: Don't reuse TLS session if the connection's SNI differs

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.

But it is only the visible part of the bug. If the SNI of the cached TLS
session does not match the one of the new connection, no reuse must be
performed at all.

To fix the bug, hash of the SNI of the reused session is compared with the
one of the new connection. The TLS session is reused only if the hashes are
the same.

This patch should fix the issue #3195. It must be slowly backported as far
as 3.0. it relies on the following series:

  * MEDIUM: tcpcheck/backend: Get the connection SNI before initializing SSL ctx
  * MINOR: connection/ssl: Store the SNI hash value in the connection itself
  * MEDIUM: ssl: Store hash of the SNI for cached TLS sessions
  * MINOR: ssl: Add a function to hash SNIs
  * MEDIUM: quic: Add connection as argument when qc_new_conn() is called
  * BUG/MINOR: ssl: Don't allow to set NULL sni