BUG-648: Improve performance by reusing old data nodes

This patchset gives yang-data-api the chance to optimize copy
operations we execute when creating a new generation.

Change-Id: Ib23a87e116a0a04f7fd4f2b709d7239e213ea744
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-648: Improve performance by reusing old data nodes"