DataTreeTip forms a point where a modification can be applied to. This
interface can be implemented by DataTreeCandidate instances (as
DataTreeCandidateTip), so users can prepare a sequence of modifications
for commit without actually applying them.
Change-Id: Ic772fa9b813105f3731c6b7cea45b629f9137de1
Signed-off-by: Robert Varga <rovarga@cisco.com>