evolve: introduce function to create a obsmarker relation even for public cset
To create obsmarkers we use obsolete.createmarkers() function, but because of
security reasons this function refuses to create obsmarkers for public cset.
And we need to create obsmarkers for a public cset while solving public content
divergence. So introducing this function which create a obsmarker relation even
for immutable cset.
Currently this function create obsmarker for a single relation, in contrast of
obsolete.createmarkers() which create markers for multiple relations.
Upcoming pathces will be using this function.