Bump yangtools to 3.0.0

Bump yangtools to 3.0.0

This is bumps yangtools to 3.0.0, adjusting for changed APIs. The
changes specifically include:

- use ListenerRegistry.getRegistrations() instead of it implementing
  Iterable
- adjust for NormalizedNodeStreamWriter interface change
- remove use of *AttrBuilder, as NormalizedNodes no longer hold
  attributes
- DataTreeCandidateNodes.empty() instead of home-grown implementation
- adjust for DataTreeCandidateNode.getModifiedChild() returning Optional
- use QueuedNotificationManager.BatchedInvoker

Change-Id: Ia30d73ccf6380e3759414b3026b822df0950a481
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>