[core] resource-directory must implement lookup interface

[Christian Amsüss]

Hello CoRE working group,

in the development of the resource-directory draft, it is the authors'
opinion that contrary to previous drafts, the lookup should not be an
optional interface. Both "sides" of the resource directory are designed
to be usable by constrained devices, are essential components of the RD
when constrained nodes are supposed to interact with each other. Leaving
lookup optional would lead to incompatibility between constrained nodes.

Note that this would not mandate that all the information in the RD must
actually be exposed; the lookup interface can (and actually SHOULD) be
subject to access restrictions up to the extent of always presenting
emtpy results to requesters.

Please let us know if you know of good reasons not to do this change.

Best regards
Christian