Move implementation of DataTree to Yangtools

Analysis showed that implementation of DataTree
in MD-SAL Data Store, is generic and can
be used outside of Data Store as part of
YANG Data component. This patch
moves existing code created by
 - Robert Varga <rovargs@cisco.com>
 - Tony Tkacik <ttkacik@cisco.com> 

To new location

yang-data-api - Data structure & data tree API definitions
yang-data-impl - Data structures and data tree logic

Change-Id: I0a93714677b5315534630d942e35e0262fa8d41d
Signed-off-by: Basheeruddin Ahmed <syedbahm@cisco.com>
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "Move implementation of DataTree to Yangtools"