Should the hash tables in circuitmux_t have channel ID in them?

(per discussion with nickm and skruffy in #tor-dev on 2015-02-02)

Arguably using (channel_id, circuit_id) pairs rather than bare circuit IDs in the hash table in a circuitmux (circuitmux_t.chanid_circid_map) is generality we never actually used and we should consider whether we can eliminate it and save memory.