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.

obsstore: keep track of children information We use the new `parents` field to build a dictionary of markers that touch children of a node. This will be used to link prune markers to a set of exchanged nodes.