[PATCH 0 of 3 V2] V2 with config option plainly dropped

[Pierre-Yves David]

After discussion with Matt Mackall, we drop the config option and enables the
advances behavior by default.

This new behavior only applies when obsolescence feature is enabled, something
current pretty hard to achieve if you don't really want to.

Discussion of the default behavior we want at the end is added to the sprint
agenda.