MEDIUM: quic/ssl: add new ex data for quic_conn

MEDIUM: quic/ssl: add new ex data for quic_conn

Allow to register quic_conn as ex-data in SSL callbacks. A new index is
used to identify it as ssl_qc_app_data_index.

Replace connection by quic_conn as SSL ex-data when initializing the QUIC
SSL session. When using SSL callbacks in QUIC context, the connection is
now NULL. Used quic_conn instead to retrieve the required parameters.
Also clean up

The same changes are conducted inside the QUIC SSL methods of xprt-quic
: connection instance usage is replaced by quic_conn.