Merge branch 'ps/odb-cleanup'

Merge branch 'ps/odb-cleanup'

Various code clean-up around odb subsystem.

* ps/odb-cleanup:
  odb: drop unneeded headers and forward decls
  odb: rename `odb_has_object()` flags
  odb: use enum for `odb_write_object` flags
  odb: rename `odb_write_object()` flags
  treewide: use enum for `odb_for_each_object()` flags
  CodingGuidelines: document our style for flags