BUG-2470: implement InMemoryDataTree.toString()

For debugging purposes, we need a way to dump the current state of the
data tree. Implement proper toString() method.

Change-Id: I808b83b1e89eaa79d2a8d19b2e551884e7a3dc50
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-2470: implement InMemoryDataTree.toString()"