obsolete: sort obsmarkers during exchange
Because bundle2 allows a more precise exchange of obsmarkers during pull, it
sends them in a different order (previously unstable because of sets.) As
a result, they are added to the repository in a different order. To stabilize
the order and ensure tests are unchanged when moving from bundle1 to bundle2 we
sort markers when exchanging them.
In the long run, the obsstore will probably not use a linear storage.