Implement additional record types

The records written in the segment store should be augmented with additional types. In OAK-2498 the following additional types were identified:

• List of property names. A list of strings, where every string is a property name, is referenced by the template record.
• List of list of values. This list is pointed to by the node record and contains the values for single- and multi- value properties of that node. The double indirection is needed to support multi-value properties.
• Map from string to node. This map is referenced by the template and represents the child relationship between nodes.
• Super root. This is a marker type identifying top-level records for the repository super-roots.

Just adding these types doesn't improve the situation for the segment store, though. Bucket and block records are not easily parseable because they have a variable length and their size is not specified in the record value itself. For record types to be used effectively, the way we serialize certain kind of data has to be reviewed for further improvements.