GET DataNode API sometimes does not return the data node from the root. 

Steps to reproduce

1. create two new anchors
2. create data node for both anchors ( bookstore.json from cps-services)
3. Send patch request for one of the anchors 
4. GET Node API for the updated anchors returns information about one for the categories.

Root cause: The current implementation fetches the first row from the fragment table which belongs to the provided dataspace and anchor. There is no guarantee that the first row will be the root and which leads to this issue.

This issue was found when testing integration between CPS and CPS temporal https://gerrit.onap.org/r/c/cps/+/121905