NIL_REFERENCES: Added mechanism to cut (nil) references during commit

*Note:* This only works with a modified GemStone version (HPI-GSS matthias/nil-references).

This is different from ```db_instances_transient```. Transient instances are committed but their instance variables are set to ```nil```. Classes that are marked as ```db_nil_references``` and their instances are not committed at all. References are replaced by ```nil```.

```ruby
class C
end

C.maglev_nil_references true
c = C.new
Maglev::PERSISTENT_ROOT[1] = [[c]]
Maglev.commit_transaction

Maglev::PERSISTENT_ROOT[1]
=> [[nil]]
c
=> #<C:0x8fb0801>
```