Local hub prefix discovery

Some applications need to discover the prefix(es) under which they can publish data / which Interests the local hub will be able to forward down to the application. In order to discover that, an application can send an Interest for /localhop/nfd/rib/routable-prefixes. The data in response to this Interest will contain a list of prefixes and should be encoded as follows:

Response = DATA-TYPE TLV-LENGTH
             Name     ; /localhop/nfd/rib/routable-prefixes/[version]/[segment]
             MetaInfo ; ContentType == BLOB, FreshnessPeriod == 5000
             ResponseContent
             DataSignature

ResponseContent = 1*Name

ResponseContent should contain at least one Name, which should be routable towards the face from which the request was received. The requester may process the list of returned names and pick whichever it wants to use.

For now, the /localhop/nfd/rib/routable-prefixes data is served by ndn-autoconfig-server.