feat(store): a repository format version, with migrations #56

Packs, operation log segments and the index each start with their own format and version. The repository as a whole has no version, so a newer origin can't tell an old layout from a damaged one, and can't upgrade it.

Record the repository's format version in .origin/. An origin that finds an older version migrates it, durably and resumable after a crash, and one that finds a newer version refuses with a clear error. This applies to local repositories as much as to servers.

Related: #20.