Implement optimistic direct references in DB

[tzaeschke]

Currently, references between objects use the OID to perform a lookup in the OID-index to find the target objects' location in the DB.
For rarely changing objects (configurable?) we could also store directly the pageID/txID of the target object. If the target page has the correct txID, then we can directly load the object without going through the OID-index. When the txId mismatches, we still have to go through the OID index.

Problems:

• This requires 8+4 additional bytes per reference. This could be made configurable on class level, i.e. we store a flag on the page that indicates whether direct optimistic references are stored or not.
• Currently, this would only help during navigation in queries. To make it work with navigation in the client, we would need to store the pageId/txId together in the materialized objects.