obsolete: write obsolete marker inside a transaction Marker are now written as soon as possible but within a transaction. Using a transaction ensure a proper behavior on error and rollback compatibility. Flush logic are not necessary anymore and are dropped from lock release. With this changeset, the obsstore is open, written and closed for every single added marker. This is expected to be highly inefficient and batched write should be implemented "quickly". Another issue is that every flush of the file will invalidate the obsstore filecache and trigger a full re instantiation of the repo.obsstore attribute (including, reading and parsing entry). This is also expected to be highly inefficient and proper filecache operation should be implemented "quickly" too. A side benefit of the filecache issue is that repo.obsstore object is properly invalidated on transaction abortion.