Update QNameFactory caching

This eliminates the use of soft references in favor of weak ones,
so that we do not rely on GC heuristics for retention.

We also make the maximum cache size runtime-configurable via
org.opendaylight.controller.cluster.datastore.node.utils.qname-cache.max-size
property.

Finally we use QName.intern() to defer to the global QName cache,
resulting in better QName instance sharing.

JIRA: CONTROLLER-1897
Change-Id: Ice3596d23b96c4ecfc5775cb21968233b042a984
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>