This reverts commit 18f2ccd244f555348b0d5920e3b71286f1e7b04b.
Found issues related to QUIC fast-forward were resolved (see github
issue #2372). Reenable it by default. If any issue arises, it can be
disabled using the global statement :
tune.quit.zero-copy-fwd-send off
This can be backported to 2.9, but only after a sensible period of
observation.