BUG/MEDIUM: mux_quic: account discarded data in connection flow control

BUG/MEDIUM: mux_quic: account discarded data in connection flow control

Read data not yet consumed by the upper stream layer are discarded when
a RESET_STREAM is received. This patch implements their accounting in
the connection flow control.

This feature is mandatory as it could result in discrepancies between
each peers for the measured connection flow control. If this grows too
big, the sender may be indefinitely blocked, forcing the opening of a
new connection.

This patch introduces a new function qcc_account_conn_fctl(), which is
used to factorize the code already present during STREAM frames
handling. This is now also called for RESET_STREAM frames.

This must be backported up to 2.8, after a period of observation.