[WIP] lib, mgmtd: Fix the Scratch Buffer usage in Northbound code

[pushpasis]

The changes to the candidate data trees were not getting added to the scratch-buffer since lyd_diff_get_op was returning nothing ('n') as no 'operational' metadata associated with the data items in the YANG models. Fixed the logic to add them to the scratch buffer anyways.

Also, the scratch-buffer updations were getting invoked everywhere (mgmtd, backend clients). Limited the usage to mgmtd daemon only.