code: def _mint_new_ott_ids(self, how_many=1):
          
          first_minted_id = self._next_ott_id
          self._next_ott_id = first_minted_id + how_many
          content = u'{"next_ott_id": %d}\n' % self._next_ott_id
          # The content is JSON, but we hand-rolled the string above
          #       so that we can use it as a commit_msg
          self._write_master_branch_resource(content,
                                             self._id_minting_file,
                                             commit_msg=content,
                                             is_json=False)
          last_minted_id = self._next_ott_id - 1
          return first_minted_id, last_minted_id
docs: ASSUMES the caller holds the _doc_counter_lock !
          Checks the current int value of the next ottid, reserves a block of
          {how_many} ids, advances the counter to the next available value,
          stores the counter in a file in case the server is restarted.
          Checks out master branch as a side effect.