MAJOR: ssl: use the msg callback mecanism for backend connections

MAJOR: ssl: use the msg callback mecanism for backend connections

Backend SSL connections never used the ssl_sock_msg_callbacks() which
prevent the use of keylog on the server side.

The impact should be minimum, though it add a major callback system for
protocol analysis, which is the same used on frontend connections.

https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set_msg_callback.html

The patch add a call to SSL_CTX_set_msg_callback() in
ssl_sock_prepare_srv_ssl_ctx() the same way it's done for bind lines in
ssl_sock_prepare_ctx().