BUG-2673: Introduced new more low-level DOM Data Change APIs

As it turned out abstraction provided by Helium Data Change
API was CPU costly and calculated lot of unnecessary data,
and did not provide necessary navigation for some use-cases.

This new API contract defines more low-level data change
APIs, where application is more in control, how it will
process / consume / decompose Data Change by providing
DataTreeCandidate.

Change-Id: I468c9583c7999bc02101a18a8adf5dc1a084f1d9
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>