Seal the dirclient::Requestable trait and hide most of its members.

This is a follow-on from !1675 (merged); it is based on that branch. The first new commit here is "dirclient: Rename partial_docs_ok to partial_response_body_ok".

In this MR, I seal the Requestable trait, and move most of its members to a the hidden supertrait.

Since none of these methods were invoked from outside tor-dirclient (except for debugging), and since we have had a fair amount of churn on what we actually want those methods to be, it seems like a good idea to use this trick to hide them. This will let us make other changes to the actual behavior of Requestable in the future.