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