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