Complain if a Conduit Client doesn't send a host key.

Summary: There is a TODO here that is a few years old... the Conduit Protocol is now at version 7.

Test Plan: One less TODO in the codebase.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9694