Only put streams onto isolation-compatible circuits
This is the meat of Proposal 171 (legacy/trac#1865 (moved)): Once we have the code to tell Tor how to isolate streams (legacy/trac#3515 (moved)), and once we have the logic to decide how to treat isolated streams (legacy/trac#3516 (moved)), we can actually hook everything up so that streams never get attached to circuits with other incompatible streams.