Optimize NodeIdentifier reading

NodeIdentifiers are just an alias for QName, which is heavily
reused. This patch adds a secondary cache for interpreting QNames
as NodeIdentifiers, with instance reuse.

This allows us to reduce the number of NodeIdentifier instances,
while not relying on NodeIdentifier.create() for the common lookup
case.

JIRA: CONTROLLER-1898
Change-Id: Ifa0c5d572f7d39da49707529c7ddd9c9f36f2dab
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>