Enable gateway discovery service to support a centralized discovery controller

[sdpeters]

An NVMe centralized discovery controller (CDC) serves the same purpose as a top-level discovery server (#71), but improves on that by eliminating the need for every host to completely enumerate all the discovery controllers on the network to find the namespaces they can/must attach to.

A centralized discovery controller interacts with "direct discovery controllers" (like the gateway's discovery service #63) in a new way recently added to the NVMe spec.

This feature provides the support in the gateway discovery service that enables it to act as a DDC to a CDC. This requires specific support in the DDC.

How widely adopted CDCs will be remains TBD. As much as anything this feature ticket is to distinguish this from #71 (which is much simpler).