lib, tests: major rework in the operational-data callbacks

lib, tests: major rework in the operational-data callbacks

The northbound infrastructure for operational data was subpar compared
to the infrastructure for configuration data. This commit addresses most
of the existing problems, making it possible to write operational-data
callbacks for more complex YANG models.

Summary of the changes:
* Add support for nested YANG lists.
* Add support for leaf-lists.
* Add support for leafs of type "empty".
* Introduce the "show yang operational-data XPATH" command, and write an
  unit test for it. The main purpose of this command is to make it
  easier to test the operational-data northbound callbacks.
* Introduce the nb_oper_data_iterate() function, that can be used
  to iterate over operational data. Make the CLI and sysrepo use this
  function.
* Since ConfD has a very peculiar API, it can't reuse the
  nb_oper_data_iterate() like the other northbound clients. In this
  case, adapt the existing ConfD callbacks to support the new features
  (and make some performance improvements in the process).

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