Merge branch 'ps/reftable-multi-level-indices-fix'

Write multi-level indices for reftable has been corrected.

* ps/reftable-multi-level-indices-fix:
  reftable: document reading and writing indices
  reftable/writer: fix writing multi-level indices
  reftable/writer: simplify writing index records
  reftable/writer: use correct type to iterate through index entries
  reftable/reader: be more careful about errors in indexed seeks