obsstore: add relevantmarkers method
We add a ``relevantmarkers`` method to fetch all markers that seem relevant to a
set of nodes. See function documentation about how this set is computed. This
will let us exchange only the markers that seem "relevant" to the set of
changesets related to a push or a pull.
The approach used to define "relevant" has been successfully tested in evolve
for 6 months.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.