obsmarker: write a message with the number of markers added through bundle2

bundle2: add an obsmarkers part handler This part contains a binary stream of obsolescence markers. Received markers are added to the repository.

obsolete: make encodemarkers a public function We'll need access to it for bundle2.

obsolete: move _encodemarkers next to _readmarkers

obsstore: store and preserve ondisk version

obsolete: have _readmarkers return the format version readmarkers is not returning the version of the format it read from. This will let callers know about the format and allow them to preserve it.

obsolete: support for any known obsstore format when reading or writing We can now read and write any known format. The list of known formats currently has one element (0). The obsstore itself is not aware of multiple formats yet and always uses format 0.