This seems odd; The segment value stuff seems specific to the table value generation strategy, I don't think this should leak through `OgmIdentityGenerator` (actually it seems not right to me anyways that `OgmIdentityGenerator` basically just is a table generator).

More generally, do we actually support generation strategies other than sequences in Neo4j? If not I think we should raise an exception when explicitly using something other than `GenerationType.AUTO` or `SEQUENCE`.