MINOR: quic: process_timer() rework

MINOR: quic: process_timer() rework

Add QUIC_FL_CONN_RETRANS_NEEDED connection flag definition to mark a quic_conn
struct as needing a retranmission.
Add QUIC_FL_PKTNS_PROBE_NEEDED to mark a packet number space as needing a
datagram probing.
Set these flags from process_timer() to trigger datagram probings.
Do not initiate anymore datagrams probing from any quic encryption level.
This will be done from the I/O handlers (quic_conn_io_cb() during handshakes and
quic_conn_app_io_cb() after handshakes).