relational database serialization

[michmech]

This serialization makes use of the following changes we agreed at the last meeting:

• language is renamed to langCode where appropriate
• relationType.scope is renamed to relationType.relationScope, it’s optional, with no default
• memberType.memberType is renamed to memberType.type
• memberType.action is renamed to memberType.hint and is optional
• labelTypeTag can have sameAs

These changes are implemented in the relational database serialization section but not in the rest of the document (yet).