The `Head` election now works in a bottom-up fashion (child regions are
processed before the parent ones), and ensures that the `Head` election
process is coherent across multiple nested regions.
This means that:
1) The late entries are coherent between the nested regions. If a node
is selected as a late entry for a region, the parent region must
follow this decision too.
2) If a node which is a `Head` candidate for a parent region, is
selected as the `Head` for a child region, the same choice must be
performed for the parent region too.
Andrea Gussoni
646ad4f97a