Add service reference binding to config registry.

Add service reference binding to config registry.

Add a database for persisting service references between commits. In netconf modules
are wired together via link in /services/service xml subtree. Each service has an
xpath pointer back to a /modules/module that implements the service. Consumer of a
service then maintains a link to the service via its namespace and reference name.
This commit fixes problem of reference names not surviving commits.

Change-Id: I784f99617bb0072fc4dc3762a28e30f9b9bcf111
Signed-off-by: Tomas Olvecky <tolvecky@cisco.com>