MEDIUM: ssl: remove connection from msg callback args

MEDIUM: ssl: remove connection from msg callback args

SSL msg callbacks are used for notification about sent/received SSL
messages. Such callbacks are registered via
ssl_sock_register_msg_callback().

Prior to this patch, connection was passed as first argument of these
callbacks. However, most of them do not use it. Worst, this may lead to
confusion as connection can be NULL in QUIC context.

This patch cleans this by removing connection argument. As an
alternative, connection can be retrieved in callbacks if needed using
ssl_sock_get_conn() but the code must be ready to deal with potential
NULL instances. As an example, heartbeat parsing callback has been
adjusted in this manner.