MEDIUM: ssl: stop using conn->xprt_ctx to access the ssl_sock_ctx

MEDIUM: ssl: stop using conn->xprt_ctx to access the ssl_sock_ctx

The SSL functions must not use conn->xprt_ctx anymore but find the context
by calling conn_get_ssl_sock_ctx(), which will properly pass through the
transport layers to retrieve the desired information. Otherwise when the
functions are called on a QUIC connection, they refuse to work for not
being called on the proper transport.