This started as a small effort to simplify the usage of "ready" channel
in nfd-master. It extended into a wider simplification/unification of
the channel usage.
Hook into the same logic already exercised for NodeResourceTopology
objects: GC watches for node delete events and immediately drops stale
objects (NRT and now also NF). In addition there is a periodic resync to
catch any missed node deletes, once every hour by default.
