[Integration Test] DataNodeMVPTest

[bbulgu]

Test inspired by DataNodeTest, with the following differences:

• Instead of a LocalSkipGraph we use an actual skip graph similar to mvp test.
• We insert data nodes in the Skip Graph (concurrently each insertion in a separate thread).
• Instead of just checking for consistency, we search for inserted data nodes concurrently (each search in a thread), and see if the result is correct.