Implement forward-records

[markmeeus]

In the current implementation, the index entries point directly to the records location. If the data moves to a new location, the index entry needs to be updated.

After indexed attributes are implemented, this approach will force us to update every index entry when an object moves. Instead, we should have the indexes point to a 'forward record' which contains the current address of the record. This causes an extra read for every index lookup, but limits the writes to this single record when the record moves.

This needs to be implemented only for data-records.