lib: avoid expensive operations when editing a candidate config

lib: avoid expensive operations when editing a candidate config

nb_candidate_edit() was calling both the lyd_schema_sort() and
lyd_validate() functions whenever a new node was added to the
candidate configuration. This was done to ensure the candidate
is always ready to be displayed correctly (libyang only creates
default child nodes during the validation process, and data nodes
aren't guaranteed to be ordered by default).

The problem is that the two aforementioned functions are too
expensive to be called in the northbound hot path. Instead, it makes
more sense to call them only before displaying the configuration
(in which case a recursive sort needs to be done). Introduce the
nb_cli_show_config_prepare() to achieve that purpose.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>