DOMMountPointService needs to perform deeper proxying

As noted during migration of restconf to MD-SAL APIs, we have a slight issue when accessing mount points between users of Controller and MD-SAL APIs.

While the mount points themselves work okay, their services are incompatible: if the mount point registers an MD-SAL DOMDataBroker, users of the controller API will not be able to resolve the corresponding service and vice-versa.

This leads to a breakage if we migrate NETCONF or RESTCONF only - suddenly RESTCONF won't be able to route the requests properly.

The proper fix is to instantiate proxy services providing two-way translation, so that both MD-SAL and Controller services are available from point points accessed via the controller API.