Unify (DOM)DataTreeIdentifier layout

DOMDataTreeIdentifier and DataTreeIdentifier are almost equivalent, except their instantiation differs – for the worse, as DOMDataTreeIdentifier exposes its constructor.

We should really tie them together with an mdsal.common.api construct:

public interface LogicalDatastorePath<T extends LogicalDatastorePath<T, P>, P extends HierarchicalIdentifier<P>>
        extends HierarchicalIdentifier<T> {
    LogicalDatastoreType datastore();

    P path();

    @Override
    default boolean contains(final T other) {
        return datastore() == other.datastore() && path().contains(other.path());
    }
}

This will pave the way towards evolving these constructs, as now they have a common property: they bind a LogicalDatastoreType with their domain's instance-identifier representation.