lib: remove leaf-list xpath hack from northbound

lib: remove leaf-list xpath hack from northbound

Currently, when editing a leaf-list, `nb_candidate_edit` expects to
receive it's xpath without a predicate and the value in a separate
argument, and then creates the full xpath. This hack is complicated,
because it depends on the operation and on the caller being a backend or
not. Instead, let's require to always include the predicate in a
leaf-list xpath. Update all the usages in the code accordingly.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>