More section definition information in section metadata

Currently, we only have a list of qualified names for all section definitions used in an archive stored in metadata.sections. For issues such as #1020 (closed) and #1181 (closed), we need more complex data.

• add a new sub-section called metadata.section_defs (we deprecate the old metadata.sections after a transition period)
• these sub section should contain keys for
  • definition as qualified name
  • the hash based definition id
  • the qualified name of all compatible definitions (i.e. the definition itself and its inherited classes)
• all these should be keyword indexed in elastic search

This will be a prerequisite for putting schemas into bundles (#1181 (closed))