code: public static void checkVersion(DB db, StoreVersion newversion, ObjectMapper mapper) throws
        IOException {
      byte[] bytes = db.get(StoreVersion.KEY);
      if (bytes == null) {
        storeVersion(db, newversion, mapper);
      } else {
        StoreVersion version = mapper.readValue(bytes, StoreVersion.class);
        if (version.major != newversion.major) {
          throw new IOException("cannot read state DB with version " + version + ", incompatible " +
              "with current version " + newversion);
        }
        storeVersion(db, newversion, mapper);
      }
    }
docs: Simple major.minor versioning scheme.  Any incompatible changes should be across major
  versions.  Minor version differences are allowed -- meaning we should be able to read
  dbs that are either earlier *or* later on the minor version.