lib: reduce memory allocation when processing large config transactions

lib: reduce memory allocation when processing large config transactions

Remove the xpath field from the nb_config_cb structure in order
to reduce its size. This allows the northbound to spend less time
allocating memory during the processing of large configuration
transactions.

To make this work, use yang_dnode_get_path() to obtain the xpath
from the dnode field whenever necessary.

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