MINOR: quic: limit usage of ssl_sock_ctx in favor of quic_conn

MINOR: quic: limit usage of ssl_sock_ctx in favor of quic_conn

Continue on the cleanup of QUIC stack and components.

quic_conn uses internally a ssl_sock_ctx to handle mandatory TLS QUIC
integration. However, this is merely as a convenience, and it is not
equivalent to stackable ssl xprt layer in the context of HTTP1 or 2.

To better emphasize this, ssl_sock_ctx usage in quic_conn has been
removed wherever it is not necessary : namely in functions not related
to TLS. quic_conn struct now contains its own wait_event for tasklet
quic_conn_io_cb().

This should be backported up to 2.6.