Reuse child NodeIdentifier

Reuse child NodeIdentifier

We effectively create two YangInstanceIdentifiers for the same thing,
except for routes we end up creating a different instance of
NodeIdentifier, which forces a slower comparison. Reuse the same
identifier, lowring memory pressure and increasing performance slightly.

Change-Id: I80e8ffa89737476040727f45a8b90bd157527955
Signed-off-by: Robert Varga <rovarga@cisco.com>
(cherry picked from commit 6416127e56a0fc0f2645823e082a4eaa39142a28)