MINOR: quic: Move the packet number space status at quic_conn level

MINOR: quic: Move the packet number space status at quic_conn level

As the packet number spaces and encryption level are dynamically allocated,
the information about the packet number space discarded status must be kept
somewhere else than in these objects.

quic_tls_discard_keys() is no more useful.
Modify quic_pktns_discard() to do the same job: flag the quic_conn object
has having discarded packet number space.
Implement quic_tls_pktns_is_disarded() to check if a packet number space is
discarded. Note the Application data packet number space is never discarded.