BUG-7464: Refactor KVNode

KVNode is not really useful, as it acts like a factory method
for turning SNode/TNode into an Map.Entry.

We do not compare those nodes based on identity, hence we can
safely make them implement immutable version of Entry interface.

In order to keep some safety, we retain the interface, renamed
to EntryNode, extending Map.Entry and providing a default
setValue() implementation.

This has the added benefit of not needing object allocation
during iteration.

Change-Id: I27e3049739f4eb97ee76e8462c9ddde3c4b5b17a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>