Expose the branch feature from NodeStore

As discussed on oak-dev@, the current NodeStateBuilder implementation in .oak.kernel is complicated by the fact that it needs to maintain a fairly complicated mapping between the uncommitted transient state kept in the builder context and the underlying persisted/branched state of the content tree.

To avoid such complications, we should expose the branch feature and allow especially copy and move operations to be performed directly against a branch
instead of indirectly through the NodeStateBuilder interface.