lib: register dependency between control plane protocol and vrf nb nodes

lib: register dependency between control plane protocol and vrf nb nodes

When the control plane protocol is created, the vrf structure is
allocated, and its address is stored in the northbound node.
The vrf structure may later be deleted by the user, which will lead to
a stale pointer stored in this node.

Instead of this, allow daemons that use the vrf pointer to register the
dependency between the control plane protocol and vrf nodes. This will
guarantee that the nodes will always be created and deleted together, and
there won't be any stale pointers.

Add such registration to staticd.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>