MINOR: quic: improve sending API on retransmit

MINOR: quic: improve sending API on retransmit

qc_send_hdshk_pkts() is a wrapper for qc_prep_hpkts() used on
retransmission. It was restricted to use two quic_enc_level pointers as
distinct arguments. Adapt it to directly use the same list of
quic_enc_level which is passed then to qc_prep_hpkts().

Now for retransmission quic_enc_level send list is built directly into
qc_dgrams_retransmit() which calls qc_send_hdshk_pkts().

Along this change, a new utility function qel_register_send() is
defined. It is an helper to build the quic_enc_level send list. It
enfores that each quic_enc_level instance is only registered in a single
list to prevent memory issues. It is both used in qc_dgrams_retransmit()
and quic_conn_io_cb().