ConnectionManager should have a way to discover approximately what the value of RequestableChannelClasses will be on connections - basically, the intersection of what's in the code, and what's in the protocol, *before* intersecting with the capabilities of the particular server we're connecting to (which can't be done until connected).

(We could consider adding Protocol objects, to avoid the "unique IDs that aren't objects" anti-pattern; or not, to save round-trips for static data retrieval. GetProtocolAttributes?)