This adds the final bits to enable CachingNormalizedNodeSerializer
to actually cache leaf objects.
JIRA: MDSAL-407
Change-Id: I39bac36395f4e6809342d48a375d441b49cbc3b8 Signed-off-by: Jie Han <han.jie@zte.com.cn> Signed-off-by: Robert Varga <robert.varga@pantheon.tech>