datastore: fix leftover from switch to get() callbacks

Signed-off-by: Zvonimir Fras <zvonimir.fras@sartura.hr>
datastore: add update() callback support

If exists, this callback will be called every time get*() traverses
through that node in the tree. With this callback, plugin developer can
update larger number of node values.

Signed-off-by: Zvonimir Fras <zvonimir.fras@sartura.hr>
ietf-system: add dummy update callback

Signed-off-by: Zvonimir Fras <zvonimir.fras@sartura.hr>