MINOR: quic: Post handshake packet building improvements

MINOR: quic: Post handshake packet building improvements

Make qc_prep_hdshk_pkts() and qui_conn_io_cb() handle the case
where we enter them with QUIC_HS_ST_COMPLETE or QUIC_HS_ST_CONFIRMED
as connection state with QUIC_TLS_ENC_LEVEL_APP and QUIC_TLS_ENC_LEVEL_NONE
to consider to prepare packets.
quic_get_tls_enc_levels() is modified to return QUIC_TLS_ENC_LEVEL_APP
and QUIC_TLS_ENC_LEVEL_NONE as levels to consider when coalescing
packets in the same datagram.