`connection_edge_compatible_with_circuit` prevents a stream with `ISO_STREAM` set from being associated to any circuit to which a stream (not necessarily a different stream) has already been associated. Nothing else looks at `ISO_STREAM`. (In other news, `connection_edge_streams_are_compatible` is dead code.) This treatment of `ISO_STREAM` should cause excess circuit creation (legacy/trac#3632) during anonymous directory operations, and it does not (explicitly) prevent a circuit which has had an `ISO_STREAM` stream attached to it from having a non-`ISO_STREAM` stream attached to it later.