DATAMONGO-1194 - Improve DBRef resolution for collections.

[christophstrobl]

We now bulk load collections of referenced objects as long as they are stored in the same collection. This reduces db roundtrips and network traffic.

---

Benchmark compare fetching list of dbrefs before and after this change:

        |    10    |   100   |   1000  |  // nr referenced Address objects
--------+----------+---------+---------+            
before  | 1317 /s  |  147 /s |   13 /s |  
after   | 3880 /s  |  776 /s |   38 /s |

class Customer {
    @Id String id;
    String name;
    @DBRef List<Address> list;
}

class Address {
    @Id String id;
    String city, street;
}