Define a new settings tune.quic.frontend.max-tot-window. It contains a
size argument which can be used to set a limit on the sum of all QUIC
connections congestion window. This is applied both on
quic_cc_path_set() and quic_cc_path_inc().
Note that this limitation cannot reduce a congestion window more than
the minimal limit which is set to 2 datagrams.
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