[PATCH 0 of 6 STABLE] Several fix for checkheads and bookmarks alteration from obsolescence

[pierre-yves.david at logilab.fr]

This series initially tries to fix issues 3561

In the process I found several bugs regarding usage of obsolescence in
checkheads. The first half of this series is dedicated to testing and fixing
thoses issue and the two last patches solve issue 3561 itself.

The changes may looks a bit bold for the freeze, but they only fix obsolete only
code path of extra some logic that were unwrappable by any third parti code.

All issues fixed are very are to workaround from outside core and it would be
very great if this series were accepted during the freeze.

issues 3561 prevent the use of bookmark with obsolescence. This is very
unfortunate as obsolescence should help to boost bookmark usage.