[core] overwrite storage strategy for entity references

[CrystalMethod]

It shall be possible to overwrite the default storage strategy for entity references. By default XO will translate such references into relationships. Most (Nosql) database don't support "real" relationships but other strategies like foreign keys. In MongoDb one can use manual refences by id or so called dbrefs (db + collection + id).

interface A {
  B getB();
  setB(B b);
}

{
"_id": original_id,
"b": "_some_manual_key"
}

{
"_id": original_id,
"b": {
  "$ref" : "b",
  "$id" : ...,
  "$db" : "db"
  }
}