BUG/MEDIUM: quic: fix crash on MUX send notification

BUG/MEDIUM: quic: fix crash on MUX send notification

MUX notification on TX has been edited recently : it will be notified
only when sending its own data, and not for example on retransmission by
the quic-conn layer. This is subject of the patch :
  b29a1dc2f4a334c1c7fea76c59abb4097422c05c
  BUG/MINOR: quic: do not notify MUX on frame retransmit

A new flag QUIC_FL_CONN_RETRANS_LOST_DATA has been introduced to
differentiate qc_send_app_pkts invocation by MUX and directly by the
quic-conn layer in quic_conn_app_io_cb(). However, this is a first
problem as internal quic-conn layer usage is not limited to
retransmission. For example for NEW_CONNECTION_ID emission.

Another problem much important is that send functions are also called
through quic_conn_io_cb() which has not been protected from MUX
notification. This could probably result in crash when trying to notify
the MUX.

To fix both problems, quic-conn flagging has been inverted : when used
by the MUX, quic-conn is flagged with QUIC_FL_CONN_TX_MUX_CONTEXT. To
improve the API, MUX must now used qc_send_mux which ensure the flag is
set. qc_send_app_pkts is now static and can only be used by the
quic-conn layer.

This must be backported wherever the previously mentionned patch is.