Merge branch 'hn/reftable-no-empty-keys'

General clean-up in reftable implementation, including
clarification of the API documentation, tightening the code to
honor documented length limit, etc.

* hn/reftable-no-empty-keys:
  reftable: rename writer_stats to reftable_writer_stats
  reftable: add test for length of disambiguating prefix
  reftable: ensure that obj_id_len is >= 2 on writing
  reftable: avoid writing empty keys at the block layer
  reftable: add a test that verifies that writing empty keys fails
  reftable: reject 0 object_id_len
  Documentation: object_id_len goes up to 31