MAJOR: htx: Enable the HTX mode by default for all proxies

MAJOR: htx: Enable the HTX mode by default for all proxies

The legacy HTTP mode is no more the default one. So now, by default, without any
option in your configuration, all proxies will use the HTX mode. The line
"option http-use-htx" in proxy sections are now useless, except to cancel the
legacy HTTP mode. To fallback on legacy HTTP mode, you should use the line "no
option http-use-htx" explicitly.

Note that the reg-tests still work by default on legacy HTTP mode. The HTX will
be enabled by default in a futur commit.