[PATCH 0 of 6] phases: publish option and pushkey sync of phase boundaries

[Pierre-Yves David]

This is a first series that lay down the base of the phase.publish option and
phase synchronization using pushkey.

The publishing concept is defined and argumented in the first changeset
description.

The goal of this series is to implement major concept and feature. There are
small details subject to changes (as for example the local behavior of
publishing changeset) and some known incorrect behavior in push to be fixed in
later series

Happy review!